  United States Court of Appeals
      for the Federal Circuit
                ______________________

           BIOSIG INSTRUMENTS, INC.,
                 Plaintiff-Appellant

                           v.

                  NAUTILUS, INC.,
                  Defendant-Appellee
                ______________________

                      2012-1289
                ______________________

    Appeal from the United States District Court for the
Southern District of New York in No. 10-CV-7722, Judge
Alvin K. Hellerstein.
                 ______________________

                Decided: April 27, 2015
                ______________________

   MARK DAVID HARRIS, Proskauer Rose LLP, New York,
NY, argued for plaintiff-appellant. Also represented by
PAUL MILCETIC, Villanova, PA; TODD KUPSTAS, Kessler
Topaz Meltzer & Check, LLP, Radnor, PA; DANIEL C.
MULVENY, Barnes & Thornburg LLP, Wilmington, DE;
JOHN E. ROBERTS, Proskauer Rose LLP, Boston, MA.

    JOHN D. VANDENBERG, Klarquist Sparkman, LLP,
Portland, OR, argued for defendant-appellee. Also
represented by JAMES E. GERINGER, PHILIP J. WARRICK,
JEFFREY S. LOVE.
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.            2
                 ______________________


 Before NEWMAN, SCHALL, and WALLACH, Circuit Judges.
WALLACH, Circuit Judge.
    This case is before us on remand from the United
States Supreme Court. Biosig Instruments, Inc. (“Biosig”)
is the assignee of U.S. Patent No. 5,337,753 (“the ’753
patent”), directed to a heart rate monitor associated with
an exercise apparatus and/or exercise procedures. Biosig
brought a patent infringement action against Nautilus,
Inc. (“Nautilus”) in district court alleging that Nautilus
infringed claims 1 and 11 of the ’753 patent. After claim
construction, Nautilus filed a motion for summary
judgment seeking, in relevant portion, to have the ’753
patent held invalid for indefiniteness. The district court
granted Nautilus’s motion, and Biosig appealed. This
court found the claims at issue were not invalid for
indefiniteness, and reversed and remanded for further
proceedings. Nautilus petitioned for certiorari, and the
Supreme Court vacated and remanded this court’s
decision. On remand, we maintain our reversal of the
district court’s determination that Biosig’s patent claims
are indefinite.
                      BACKGROUND
     The facts of this case were recited in detail in this
court’s previous opinion and need not be repeated in full
here. Nautilus, Inc. v. Biosig Instruments, Inc. (Nautilus
I), 715 F.3d 891, 898 (Fed. Cir. 2013) In summary, the
’753 patent is directed to a heart rate monitor that
purports to improve upon the prior art by effectively
eliminating “noise” signals during the process of detecting
a user’s heart rate. ’753 patent col. 1 ll. 5–10. The ’753
patent discloses an apparatus preferably mounted on
exercise equipment that measures heart rates by, inter
alia, processing electrocardiograph (“ECG”) signals from
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.             3
which electromyogram (“EMG”) signals are substantially
removed. Id. col. 1. ll. 36–41. Claim 1 is representative
and recites, in relevant part:
    1. A heart rate monitor for use by a user in
    association with exercise apparatus and/or
    exercise procedures, comprising:
    an elongate member;
    electronic circuitry including a difference
    amplifier having a first input terminal of a first
    polarity and a second input terminal of a second
    polarity opposite to said first polarity;
    said elongate member comprising a first half and
    a second half;
    a first live electrode and a first common electrode
    mounted on said first half in spaced relationship
    with each other;
    a second live electrode and a second common
    electrode mounted on said second half in spaced
    relationship with each other;
    said first and second common electrodes being
    connected to each other and to a point of common
    potential . . . .
Id. col. 5 ll. 17–36 (emphases added).
    Biosig sued Nautilus for infringement of the ’753
patent in August 2004. After several reexamination
proceedings, Biosig reinstituted a patent infringement
action against Nautilus on October 8, 2010. On August
11, 2011, the district court conducted a Markman hearing,
and on September 29, 2011, issued its order construing
certain disputed claim terms. On November 10, 2011,
Nautilus moved under Federal Rule of Civil Procedure 56
for summary judgment on two issues: infringement and
invalidity for indefiniteness. On February 22, 2012, the
district court granted Nautilus’s motion, holding the ’753
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.            4
patent’s “spaced relationship” term as recited in claim 1
was indefinite as a matter of law. The court did not
decide the issue of infringement.
    On appeal, this court reversed and remanded. Citing
precedent, we stated that a claim is indefinite “only when
it is ‘not amenable to construction’ or ‘insolubly
ambiguous.’”     Nautilus I, 715 F.3d at 898 (quoting
Datamize, LLC v. Plumtree Software, Inc., 417 F.3d 1342,
1347 (Fed. Cir. 2005)).       Under that standard, we
determined the ’753 patent survived indefiniteness
review. Considering the “intrinsic evidence,” we found
that it provided “certain inherent parameters of the
claimed apparatus, which to a skilled artisan may be
sufficient to understand the metes and bounds of ‘spaced
relationship.’” Id. at 899.
    The Supreme Court granted certiorari, 134 S. Ct. 896
(2014), and, rejecting our “not amenable to construction or
insolubly ambiguous” standard, vacated and remanded.
Nautilus, Inc. v. Biosig Instruments, Inc. (Nautilus II),
134 S. Ct. 2120 (2014). In its decision, the Court
articulated the standard to be applied: “[W]e hold that a
patent is invalid for indefiniteness if its claims, read in
light of the specification delineating the patent, and the
prosecution history, fail to inform, with reasonable
certainty those skilled in the art about the scope of the
invention.” Id. at 2124 (emphasis added).
    This court has jurisdiction pursuant to 28 U.S.C.
§ 1295(a)(1) (2012).
                       DISCUSSION
          I. Standard of Review & Legal Framework
    A patent must “conclude with one or more claims
particularly pointing out and distinctly claiming the
subject matter which the applicant regards as [the]
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.               5
invention.” 35 U.S.C. § 112 ¶ 2 (2006). 1 A claim is invalid
for indefiniteness if its language, when read in light of the
specification and the prosecution history, “fail[s] to
inform, with reasonable certainty, those skilled in the art
about the scope of the invention.” Nautilus II, 134 S. Ct.
at 2124. We review the district court’s indefiniteness
determination de novo. Interval Licensing LLC v. AOL,
Inc., 766 F.3d 1364, 1370 (Fed. Cir. 2014).
     A patent is presumed valid under 35 U.S.C. § 282 and,
“consistent with that principle, a [fact finder is] instructed
to evaluate . . . whether an invalidity defense has been
proved by clear and convincing evidence.” Microsoft Corp.
v. i4i Ltd. P’ship, 131 S. Ct. 2238, 2241 (2011).
    “In the face of an allegation of indefiniteness, general
principles of claim construction apply.” Enzo Biochem,
Inc. v. Applera Corp., 599 F.3d 1325, 1332 (Fed. Cir. 2010)
(internal quotation marks and citation omitted). “In that
regard, claim construction involves consideration of
primarily the intrinsic evidence, viz., the claim language,
the specification, and the prosecution history.”         Id.
Though the ultimate construction of a claim term is a
legal question reviewed de novo, underlying factual
determinations made by the district court based on
extrinsic evidence are reviewed for clear error. Teva
Pharm. USA, Inc. v. Sandoz, Inc., 135 S. Ct. 831, 842
(2015). In contrast, “when the district court reviews only
evidence intrinsic to the patent (the patent claims and
specifications, along with the patent’s prosecution
history), the judge’s determination will amount solely to a


    1    Paragraph 2 of 35 U.S.C. § 112 was replaced with
newly designated § 112(b) when § 4(c) of the Leahy–Smith
America Invents Act (“AIA”), Pub. L. No. 112–29, took
effect on September 16, 2012. Because the application
resulting in the patent was filed before that date, we will
refer to the pre-AIA version of § 112.
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.             6
determination of law, and the Court of Appeals will
review that construction de novo.” Id. at 841.
    When a “word of degree” is used, the court must
determine whether the patent provides “some standard
for measuring that degree.” Enzo Biochem, 599 F.3d at
1332; Seattle Box Co., Inc. v. Indus. Crating & Packing,
Inc., 731 F.2d 818, 826 (Fed. Cir. 1984). Recently, this
court explained: “[w]e do not understand the Supreme
Court to have implied in [Nautilus II], and we do not hold
today, that terms of degree are inherently indefinite.
Claim language employing terms of degree has long been
found definite where it provided enough certainty to one
of skill in the art when read in the context of the
invention.”     Interval Licensing, 766 F.3d at 1370.
Moreover, when a claim limitation is defined in “purely
functional terms,” a determination of whether the
limitation is sufficiently definite is “highly dependent on
context (e.g., the disclosure in the specification and the
knowledge of a person of ordinary skill in the relevant art
area).” Halliburton Energy Servs., Inc. v. M-I LLC, 514
F.3d 1244, 1255 (Fed. Cir. 2008).
    Prior to the Supreme Court’s decision in this case, a
claim was indefinite when it was “insolubly ambiguous” or
“not amenable to construction.” Datamize, 417 F.3d at
1347 (internal quotations and citations omitted). In
Nautilus II, the Supreme Court observed that § 112, ¶ 2
requires “a delicate balance.” 134 S. Ct. at 2128 (quoting
Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535
U.S. 722, 731 (2002)). On one hand, the Court noted, the
definiteness requirement must take into account the
inherent limitations of language. “Some modicum of
uncertainty,” the Court recognized, is the “‘price of
ensuring the appropriate incentives for innovation.’” Id.
(quoting Festo Corp, 535 U.S. at 741). On the other hand,
the Court explained, a patent must be precise enough to
afford clear notice of what is claimed, thereby “appris[ing]
the public of what is still open to them. Otherwise there
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.             7
would be a zone of uncertainty which enterprise and
experimentation may enter only at the risk of
infringement claims.” Id. at 2129 (internal quotation
marks and citations omitted).          The Court further
explained the policy rationale: “absent a meaningful
definiteness check . . . patent applicants face powerful
incentives to inject ambiguity into their claims.” Id.
    Balancing these competing interests, the Supreme
Court held that “[t]o determine the proper office of the
definiteness command, . . . we read § 112, ¶ 2 to require
that a patent’s claims, viewed in light of the specification
and prosecution history, inform those skilled in the art
about the scope of the invention with reasonable
certainty.” Id. (emphasis added). “The standard adopted”
by the Supreme Court “mandates clarity, while
recognizing that absolute precision is unattainable.” Id.
at 2129. It also accords with opinions of the Court stating
that “the certainty which the law requires in patents is
not greater than is reasonable, having regard to their
subject-matter.” Id. (quoting Minerals Separation, Ltd. v.
Hyde, 242 U.S. 261, 270 (1916) (emphasis added)).
   II. The Sole Issue Presented Here Is Indefiniteness
    On remand from the Supreme Court, the sole issue
presented to this court is whether the district court erred
in holding the ’753 patent invalid for indefiniteness. In
particular, the district court held that “spaced
relationship” as recited in claim 1, and referring to the
spacing between the common and live electrodes, was not
distinctly pointed out and particularly claimed in the
patent in violation of 35 U.S.C. § 112, ¶ 2.
    Before this court, Nautilus and Biosig dispute
whether the Supreme Court articulated a new, stricter
standard or whether, in rejecting the phrases “insolubly
ambiguous” and “amenable to construction,” the Court
was primarily clarifying that a patent’s claims must
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.              8
inform those skilled in the art with “reasonable certainty”
of what is claimed.
     Nautilus argues the Supreme Court’s mandate
requires this court to find the term “spaced relationship”
indefinite because “the original intrinsic evidence point[s]
in two opposite directions, leaving the claims’
boundaries—and thus the potential avenues for follow-on
innovation—fundamentally uncertain.” Nautilus’s Supp.
Br. 14. According to Nautilus, “spaced relationship” could
mean “a special spacing critical in some way to the recited
result” or it could mean the opposite, that it is not limited
or linked by the recited result. Id. at 14, 17.
    Biosig counters that “‘reasonable certainty’ is not a
new standard; it is the degree of clarity in patent claiming
that has governed for nearly one hundred years.” Biosig’s
Supp. Br. 3. According to Biosig, “the Supreme Court did
not indicate that [this court, in Nautilus I,] had been led
astray by either of the disapproved phrases. Its main
concern, rather, was that the use of those phrases by the
Federal Circuit could send the wrong message to district
courts and the patent bar.” Id. at 4 (citing Nautilus II,
134 S. Ct. at 2130 & n.8).
    III. Reasonable Certainty Under Nautilus II Is a
                  Familiar Standard
    As the Supreme Court emphasized in Nautilus II,
§ 112 “requires that a patent specification ‘conclude with
one or more claims particularly pointing out and
distinctly claiming the subject matter’” of the invention.
134 S. Ct. at 2124. The Court found too imprecise our
“insolubly ambiguous” standard, and instead held that “a
patent is invalid for indefiniteness if its claims, read in
light of the specification delineating the patent, and the
prosecution history, fail to inform, with reasonable
certainty, those skilled in the art about the scope of the
invention.” Id. (emphasis added).
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.                9
    The Court has accordingly modified the standard by
which lower courts examine allegedly ambiguous claims;
we may now steer by the bright star of “reasonable
certainty,” rather than the unreliable compass of
“insoluble ambiguity.”
    Reasonableness is the core of much of the common
law, and “reasonable certainty” has been defined in broad
spectra of the law. See, e.g., Palsgraf v. Long Island R.R.
Co., 162 N.E. 99 (N.Y. 1928) (explaining the “reasonable
[person]” foreseeability standard in tort); cf. Jay v. Sec’y of
Dep’t of Health & Human Servs., 998 F.2d 979, 984 (Fed.
Cir. 1993) (discussing whether a reasonable person could
conclude a certain vaccine caused the child’s death). The
Supreme Court has articulated a spectrum for
interpretation of the phrase “reasonable certainty.” 2


   2    Prior to Nautilus II, the Court discussed
“reasonable certainty” on numerous occasions. See, e.g.,
Kelo v. City of New London, 545 U.S. 469, 487–88 (2005)
(where petitioners argued the Court should require
“reasonable certainty” that expected public benefits in a
takings case would actually accrue, the Court declined to
impose that “heightened form of review”); Allentown Mack
Sales & Serv., Inc. v. Nat’l Labor Review Bd., 522 U.S.
359, 369 (1998) (express disavowals by more than half of
an employee bargaining unit would establish reasonable
certainty of union nonsupport); Coffy v. Republic Steel
Corp., 447 U.S. 191, 197–99 (1980) (explaining “there
must be a reasonable certainty that the benefit would
have accrued if the employee had not gone into the
military service”); Ala. Power Co. v. Davis, 431 U.S. 581,
587–88, 591 (1977) (reasonable certainty requirement for
accrual of benefits in veteran’s reemployment case
satisfied where work history demonstrates veteran “would
almost certainly have accumulated accredited service”);
Tilton v. Mo. Pac. R.R. Co., 376 U.S. 169, 180–81 (1964);
Boyce Motor Lines v. United States, 342 U.S. 337, 340
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.             10


(1952) (notice of conduct required by criminal statute
must be sufficiently definite to guide its application,
however “[t]he requirement of reasonable certainty does
not preclude the use of ordinary terms to express ideas
which find adequate interpretation in common usage and
understanding”) (emphasis added) (citation omitted);
United States v. Penn. Foundry & Mfg. Co., 337 U.S. 198,
207–08 (1949) (declining to accept the Court of Claims
“rough estimate” as providing the reasonable certainty
required for establishing damages); United Carbon Co. v.
Binney & Smith Co., 317 U.S. 228, 234 (1942) (“What on
first impression [in a patent infringement action] appears
to be reasonable certainty of dimension disappears when
we learn that ‘approximately one-sixteenth of an inch in
diameter’ includes a variation from approximately 1/4th
to 1/100th of an inch. So read, the claims are but
inaccurate suggestions of the functions of the product.”)
(emphasis added); Palmer v. Conn. Ry. & Lighting Co.,
311 U.S. 544, 558, 561 (1941) (“Certainty in the fact of
damage is essential. [Reasonable] [c]ertainty as to the
amount goes no further than to require a basis for a
reasoned conclusion.”); Sheldon v. Metro-Goldwyn
Pictures Corp., 309 U.S. 390, 404 (1940) (analogizing
copyright damages to patent infringement, the Court
found a reasonable certainty requirement satisfied not
with “mathematical exactness,” but only a “reasonable
approximation”); Sproles v. Binford, 286 U.S. 374, 393
(1932) (“The requirement of reasonable certainty does not
preclude the use of ordinary terms to express ideas which
find adequate interpretation in common usage and
understanding,”); Hamilton-Brown Shoe Co. v. Wolf Bros.
& Co., 240 U.S. 251, 261–62 (1916) (analogizing
trademark damages to patent damages and holding “[t]he
difficulty lies in ascertaining what proportion of the profit
is due to the trademark . . . and as this cannot be
ascertained with any reasonable certainty, it is more
consonant with reason and justice that the owner of the
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.             11
     Following the issuance of Nautilus II, this court
applied the reasonable certainty standard in DDR
Holdings, LLC v. Hotels.com, 773 F.3d 1245, 1260–61
(Fed. Cir. 2014). (after analogizing to facts from prior
cases and applying a “reasonable certainty” standard,
finding the term “look and feel” had an established
meaning in the art as demonstrated by the trial record,
thus informing those skilled in the art with reasonable
certainty). In Interval Licensing, the “reasonable
certainty” test was applied to determine whether one of
the embodiments provided “a reasonably clear and
exclusive definition,” with a focus on the “relationship”
between the embodiments and the claim language, and
whether the embodiments created “objective boundaries”
for those skilled in the art. 766 F.3d at 1371. The court
also stated:

trademark should have the whole profit than that he
should be deprived of any part of it by the fraudulent act
of the defendant”); Ont. Land Co. v. Yordy, 212 U.S. 152,
157 (1909) (“The first requisite of an adequate description
is that the land shall be identified with reasonable
certainty; but the degree of certainty required is always
qualified by the application of the rule that that is certain
which can be made certain.”) (quoting Jones on Law of
Real Property in Conveyancing § 323); Hetzel v. Balt. &
Ohio R.R. Co., 169 U.S. 26, 38 (1898) (“In many cases
[proof which excludes the possibility of a doubt] cannot be
given, and yet there might be a reasonable certainty,
founded upon inferences legitimately and properly
deducible from the evidence . . . .”); United States v.
Smith, 18 U.S 153, 160–62 (1820) (the crime of piracy is
defined by the law of nations with reasonable certainty);
cf. Bell Atl. Corp. v. Twombly, 550 U.S. 544, 578 n.5
(2007) (Stevens, J., dissenting); Boyde v. California, 494
U.S. 370, 393 n.2 (1990); Thor Power Tool Co. v. Comm’r,
439 U.S. 522, 543–44 (1979); Pub. Util. Comm’n v. United
States, 355 U.S. 534, 552 (1958).
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.                12
    We do not understand the Supreme Court to have
    implied in Nautilus [II], and we do not hold today,
    that terms of degree are inherently indefinite. . . .
    Although absolute or mathematical precision is
    not required, it is not enough as some of the
    language in our prior cases may have suggested to
    identify “some standard for measuring the scope
    of the phrase.” . . . The patents’ “unobtrusive
    manner” phrase is highly subjective, and, on its
    face, provides little guidance to one of skill in the
    art. . . . The patents contemplate a variety of
    stimuli that could impact different users in
    different ways. As we have explained, a term of
    degree fails to provide sufficient notice of its scope
    if it depends on the unpredictable vagaries of any
    one person’s opinion.
Id. at 1371–72 (citations omitted); see also Augme Techs.
v. Yahoo!, Inc., 755 F.3d 1326, 1340 (Fed. Cir. 2014) (A
limitation “clear on its face” “unquestionably meets [the
Nautilus II] standard.”).
    In the wake of Nautilus II, judges have had no
problem operating under the reasonable certainty
standard.     For example, Judge Bryson, sitting by
designation in Texas, stated: “Indefiniteness is a legal
determination; if the court concludes that a person of
ordinary skill in the art, with the aid of the specification,
would understand what is claimed, the claim is not
indefinite.” Freeny v. Apple Inc., No. 2:13-CV-00361-
WCB, 2014 WL 4294505, at *4 (E.D. Tex. Aug. 28, 2014)
(describing the question as whether “a person of ordinary
skill can discern from the claims and specification what
the bounds of the claim are with reasonable certainty”).
After listing numerous fact-specific examples, Judge
Bryson noted that “[c]ontrary to the defendant’s
suggestion, [the Nautilus II] standard does not render all
of the prior Federal Circuit and district court cases
inapplicable” and “all that is required is that the patent
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.            13
apprise [ordinary-skilled artisans] of the scope of the
invention.” Id. at *5. 3


   3     Prior to Nautilus II, a number of our cases
applied a reasonable certainty standard in various
contexts. See, e.g., ICU Med., Inc. v. Alaris Med. Sys., 558
F.3d 1368, 1375 (Fed. Cir. 2009) (discussing the
importation of limitations from the specification into the
claims, noting that “the line between construing terms
and importing limitations can be discerned with
reasonable certainty and predictability if the court’s focus
remains on how a person of ordinary skill in the art would
understand the claim terms”) (quoting Phillips v. AWH
Corp., 415 F.3d 1303, 1323 (Fed. Cir. 2005) (en banc)). In
a lost profits damages analysis, we noted the amount
“need not be proved with unerring precision” and held
that lost profits had been proved with reasonable
certainty and that a district court finding to the contrary
was clearly erroneous where the dealer profit margin was
“roughly twenty-five percent of the dealer price.” Ryco,
Inc. v. Ag-Bag Corp., 857 F.2d 1418, 1428 (Fed. Cir. 1988)
(citations omitted). In In re Clarke, 356 F.2d 987, 992
(CCPA 1966), our predecessor court held that “where it
can be concluded that facts . . . in support of a general
allegation of conception and reduction to practice . . .
would persuade one of ordinary skill in the art to a
reasonable certainty that the applicant possessed so much
of the invention as to encompass the reference disclosure,
then that showing should be accepted as establishing a
prima facie case of inventorship.”             Finally, our
predecessor noted, in Young v. Bullitt, 233 F.2d 347, 351
(CCPA 1956), that in a priority contest in a patent
interference, “[t]he question generally is whether, when
all the circumstances are considered together, there is a
reasonable certainty as to the identity of the product.
Proof beyond a reasonable doubt is not necessary where
applications were copending.”
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.             14
      IV. Biosig’s Claims Inform a Skilled Artisan With
                   Reasonable Certainty
    Considering this background, and the Supreme
Court’s articulated concerns in Nautilus II (the
necessarily inexact balance between “the inherent
limitations of language” and the “modicum of uncertainty”
which is “the price of ensuring the appropriate incentives
for innovation,” on the one hand, and, on the other,
enough precision “to afford clear notice of what is
claimed”), we conclude that Biosig’s claims inform those
skilled in the art with reasonable certainty about the
scope of the invention. Nautilus II, 134 S. Ct. at 2128–29
(internal quotation marks and citations omitted). As we
have stated in the past, “[t]he degree of precision
necessary for adequate claims is a function of the nature
of the subject matter.” Miles Labs., Inc. v. Shandon, Inc.,
997 F.2d 870, 875 (Fed. Cir. 1993) (citing Hybritech Inc. v.
Monoclonal Antibodies, Inc., 802 F.2d 1367, 1375 (Fed.
Cir. 1986)).
    On certiorari, the Supreme Court “express[ed] no
opinion on the validity of the patent-in-suit” but rather
instructed this court “to decide the case employing the
standard we have prescribed.” Nautilus II, 134 S. Ct. at
2124.
    As an initial matter, since our decision in Nautilus I,
the Supreme Court determined in Teva Pharmaceuticals
USA v. Sandoz that “when the district court reviews only
evidence intrinsic to the patent (the patent claims and
specifications, along with the patent’s prosecution
history), the judge’s determination will amount solely to a
determination of law, and the Court of Appeals will
review that construction de novo.” 135 S. Ct. at 841.
However, “when the district court looks beyond the
intrinsic evidence and consults extrinsic evidence, for
example to understand the relevant science, these
subsidiary fact findings are reviewed for clear error.”
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.               15
Enzo Biochem Inc. v. Applera Corp., No. 2014-1321, 2015
WL 1136421, at *4 (Fed. Cir. Mar. 16, 2015).
    Our prior analysis primarily relied on intrinsic
evidence and we found the “extrinsic evidence
underscores the intrinsic evidence.” Nautilus I, 715 F.3d
at 901. We revisit the intrinsic evidence here to make
clear that a skilled artisan would understand with
reasonable certainty the scope of the invention.
    In relevant part, we noted an ordinarily skilled
artisan would be able to determine this language requires
the spaced relationship to be neither infinitesimally small
nor greater than the width of a user’s hands. Specifically,
we stated:
   [T]he district court is correct that the specification
   of the ’753 patent does not specifically define
   “spaced relationship” with actual parameters, e.g.,
   that the space between the live and common
   electrodes is one inch. Nevertheless, the ’753
   patent’s claim language, specification, and the
   figures illustrating the “spaced relationship”
   between the live and common electrodes are
   telling and provide sufficient clarity to skilled
   artisans as to the bounds of this disputed term.
   For example, on the one hand, the distance
   between the live electrode and the common
   electrode cannot be greater than the width of a
   user’s hands because claim 1 requires the live and
   common electrodes to independently detect
   electrical signals at two distinct points of a hand.
   On the other hand, it is not feasible that the
   distance between the live and common electrodes
   be infinitesimally small, effectively merging the
   live and common electrodes into a single electrode
   with one detection point. See ’753 patent col. 3 ll.
   26–31 (describing how each hand is placed over
   the live and common electrodes so that they are
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.            16
   “in physical and electrical contact with both
   electrodes”).
Nautilus I, 715 F.3d at 899.
    The prosecution history further illustrates that the
term is not indefinite. In Nautilus I, we considered the
functionality of the claimed heart rate monitor as recited
in claim 1, “which provided the basis for overcoming the
PTO’s office action rejections during the reexamination.”
Id. Specifically, claim 1 provides, in part:
   whereby, a first electromyogram signal will be
   detected between said first live electrode and said
   first   common     electrode,     and    a   second
   electromyogram signal, of substantially equal
   magnitude and phase to said first electromyogram
   signal will be detected between said second live
   electrode and said second common electrode; so
   that, when said first electromyogram signal is
   applied to said first terminal and said second
   electromyogram signal is applied to said second
   terminal, the first and second electromyogram
   signals will be subtracted from each other to
   produce a substantially zero electromyogram
   signal at the output of said difference amplifier.
Id. col. 5 ll. 48–61. This “whereby” clause describes the
function of substantially removing EMG signals that
necessarily follows from the previously-recited structure
consisting of the elongate member, the live electrode, and
the common electrode. Id. col. 5 ll. 42–47. As we
described in Nautilus I,
   [t]he EMG signal is detected between the live and
   common electrodes, which are in a “spaced
   relationship” with each other.          Even more
   significantly, the PTO examiner found this
   function to be “crucial” as a reason for overcoming
   the cited prior art and confirming the
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.            17
   patentability of the asserted claims upon
   reexamination. J.A. 139–46. Thus, the recitation
   of this function in claim 1 is highly relevant to
   ascertaining the proper bounds of the “spaced
   relationship” between the live and common
   electrodes. See Hoffer v. Microsoft Corp., 405 F.3d
   1326, 1329 (Fed. Cir. 2005) (per curiam) (“[W]hen
   the ‘whereby’ clause states a condition that is
   material to patentability, it cannot be ignored in
   order to change the substance of the invention.”).
Nautilus I, 715 F.3d at 900. Not only is the recitation of
this function in claim 1 “highly relevant” to ascertaining
the boundaries of the “spaced relationship” between the
live and common electrodes, it shows
   a skilled artisan could apply a test and determine
   the “spaced relationship” as pertaining to the
   function of substantially removing EMG signals.
   Indeed, the test would have included a standard
   oscilloscope connected to both the inputs and
   outputs of the differential amplifier to view the
   signal wave forms and to measure signal
   characteristics. With this test, configurations
   could have been determined by analyzing the
   differential amplifier input and output signals for
   detecting EMG and ECG signals and observing
   the substantial removal of EMG signals from ECG
   signals while simulating an exercise.
Id. at 900–1.
    As discussed in detail in Nautilus I, during
prosecution, Biosig also presented evidence in the form of
a declaration by the inventor, Mr. Gregory Lekhtman.
See 01 Communique Lab., Inc. v. LogMeIn, Inc., 687 F.3d
1292, 1298 (Fed. Cir. 2012) (considering statements made
during reexamination as intrinsic evidence for purposes of
claim construction). Mr. Lekhtman argued that when
“configuring the claimed heart rate monitor, skilled
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.            18
artisans can determine the ‘spaced relationship’ between
live and common electrodes by calculating the point in
which EMG signals are substantially removed.” Nautilus
I, 715 F.3d at 900. As we explained, Mr. Lekhtman
testified:
      [T]he strength of an EMG signal measurement is
      proportional to the space between the active and
      ground electrode and the size of the electrodes.
      J.A. 194–95. . . . [I]t was common knowledge for
      skilled artisans in 1992 that EMG potentials on
      each hand would be different, and that the ’753
      patent requires a configuration of the detectors
      that produce equal EMG signals from the left and
      right hands. J.A. 200. This equalization or
      balancing . . . is achieved by detecting EMG
      signals on the left and right palms, which are
      delivered to a differential amplifier in the EMG
      measuring device. Available design variables are
      then adjusted until the differential output is
      minimized, i.e., close to zero, and the ECG to
      EMG ratio is determined to be sufficient for an
      accurate heart rate determination. J.A. 200–01.
Id.
    In this case, a skilled artisan would understand the
inherent parameters of the invention as provided in the
intrinsic evidence. The term “spaced relationship” does
not run afoul of “the innovation-discouraging ‘zone of
uncertainty’ against which [the Supreme Court] has
warned,” and to the contrary, informs a skilled artisan
with reasonable certainty of the scope of the claim.
Interval Licensing, 766 F.3d at 1374 (quoting Nautilus II,
134 S. Ct. at 2130).
                        CONCLUSION
    We conclude the “spaced relationship” phrase
“inform[s] those skilled in the art about the scope of the
 BIOSIG INSTRUMENTS, INC.   v. NAUTILUS, INC.     19
invention with reasonable certainty.” The claims that
include that phrase comply with Section 112 ¶2.
              REVERSED AND REMANDED
                              COSTS
   Each party shall bear its own costs.
