  United States Court of Appeals
      for the Federal Circuit
                ______________________

               BASF CORPORATION,
                 Plaintiff-Appellant

                           v.

             JOHNSON MATTHEY INC.,
                 Defendant-Appellee
               ______________________

                      2016-1770
                ______________________

    Appeal from the United States District Court for the
District of Delaware in No. 1:14-cv-01204-SLR-SRF,
Judge Sue L. Robinson.
                ______________________

              Decided: November 20, 2017
                ______________________

    DEANNE MAYNARD, Morrison & Foerster LLP, Wash-
ington, DC, argued for plaintiff-appellant. Also repre-
sented by MARC A. HEARRON, SETH W. LLOYD, DANIEL P.
MUINO.

    DOUGLAS E. MCCANN, Fish & Richardson, P.C., Wil-
mington, DE, argued for defendant-appellee. Also repre-
sented by PATRICK D. COONEY, MARTINA TYREUS HUFNAL,
ROBERT M. OAKES.
                ______________________

Before LOURIE, O’MALLEY, and TARANTO, Circuit Judges.
2                         BASF CORP.   v. JOHNSON MATTHEY INC.



TARANTO, Circuit Judge.
     BASF Corporation owns U.S. Patent No. 8,524,185,
which describes and claims systems for performing cata-
lytic conversion of nitrogen oxides (NOx) in an exhaust gas
stream. As relevant here, the patent claims a partly-dual-
layer arrangement of coatings on a substrate over which
exhaust gas passes—a coat along the full length of the
substrate containing “a material composition B effective
to catalyze selective catalytic reduction (SCR) of NOx”;
and beneath part of that coat, on the outlet end of the gas
passage, a partial-substrate undercoat containing “a
material composition A effective for catalyzing NH3
oxidation” (ammonia oxidation, or AMOx). ’185 patent,
col. 19, lines 40–55 (claim 1); see also id., col. 20, lines 3–5
(dependent claim 5, similar); id., col. 20, lines 42–62
(independent claim 17, similar, but adding restrictions
concerning precious metals). In 2014, BASF sued its
competitor, Johnson Matthey Inc., for infringement of the
’185 patent. The district court held that the “effective for
catalyzing”/“effective to catalyze” language is indefinite
and entered judgment of invalidity of all claims on that
basis.
    BASF appeals. We have jurisdiction under 28 U.S.C.
§ 1295(a)(1). We reverse the judgment of invalidity for
indefiniteness. We remand for further proceedings in
accordance with this opinion.
                               I
                               A
    The ’185 patent claims a partly-dual-layer arrange-
ment of catalytic coatings on a substrate over which
exhaust gas passes, e.g., the walls of a flow-through
chamber having a honeycomb structure, whose function is
to remove NOx from a stream of exhaust gas while mini-
mizing the amount of ammonia that ends up being re-
leased from the system. Claim 1 is representative:
BASF CORPORATION   v. JOHNSON MATTHEY INC.                3



   A catalyst system for treating an exhaust gas
   stream containing NOx, the system comprising:
   at least one monolithic catalyst substrate having
   an inlet end and an outlet end; an undercoat
   washcoat layer coated on one the outlet end of the
   monolithic substrate and which covers less than
   100% of the total length of the monolithic sub-
   strate, and containing a material composition A
   effective for catalyzing NH3 oxidation;
   an overcoat washcoat layer coated over a total
   length of the monolithic substrate from the inlet
   end to the outlet end sufficient to overlay the un-
   dercoat washcoat layer, and containing a material
   composition B effective to catalyze selective cata-
   lytic reduction (SCR) of NOx; and
   wherein material composition A and material
   composition B are maintained as physically sepa-
   rate catalytic compositions.
’185 patent, col. 19, lines 40–55. Relevantly similar
language about effective catalysis appears in claims 5 and
17, as already noted. The parties have not suggested any
distinction among the claims or their language that is
material to the point at issue. We focus on the language,
“composition . . . effective to catalyze,” but our analysis
applies equally to “composition . . . effective for catalyz-
ing.”
    The specification describes the generally contemplat-
ed two-phase operation of a partly-dual-layer, two-zone
coating system, which involves a full-length coating that
is the sole coat for part of the substrate (the first zone)
and that lies atop another layer on part of the substrate,
toward the outlet of the gas stream (the second zone).
The gas stream travels along the substrate from the inlet
to the outlet and is exposed, along the full length of the
substrate, to “material composition B,” which removes
4                          BASF CORP.   v. JOHNSON MATTHEY INC.



NOx by catalyzing an SCR reaction between NOx and
ammonia. Id., col. 11, lines 40–47. The ammonia for the
reaction may be injected into the gas upstream of the
catalysts. Id., col. 3, lines 28–29. That SCR process,
however, can leave unreacted ammonia, which, if un-
treated, might escape through the outlet of the system
along with the treated gas stream. Id., col. 1, lines 41–48.
The ’185 patent’s system addresses that problem (so-
called “ammonia slip,” id., col. 1, line 41) by use of an
undercoat layer, beneath a part of the full-length layer of
SCR catalyst, toward the outlet end of the substrate. At
that dual-layer end of the substrate, the ammonia is
exposed to the undercoat, which contains a “material
composition A” effective to catalyze an AMOx reaction,
reducing the residual ammonia, see id., col. 11, lines 47–
52; and the overcoat continues to minimize NOx via SCR,
id., col. 12, lines 37–45.
     When referring to compositions A and B, the specifi-
cation uses the language of “composition . . . effective to
catalyze” (or comparable “effective” terminology), e.g., id.,
col. 2, lines 5, 9, 22, 24; col. 3, lines 9, 14, 33, 38–39; col. 5,
lines 40, 48, 55–58, and, in ways that are interchangeable
for present purposes, the names “SCR catalyst” and
“ammonia oxidation [or AMOx] catalyst,” e.g., id., col. 1,
lines 30–58; col. 5, lines 38–49; col. 6, lines 48–55; col. 8,
lines 4–14, 37–41; col. 10, lines 56–61; col. 11, lines 9–20;
col. 11, line 65 through col. 12, line 3; col. 12, lines 13–26.
(It also uses certain other terms, such as “SCR composi-
tion” and “NH3 oxidation composition.” E.g., id., col. 7,
line 58; col. 8, line 48.) The specification sets out the
specific stoichiometric chemical reactions for the cata-
lysts. Id., col. 5, lines 33–49. It identifies a variety of
materials that can be used for “material composition A”
(e.g., “refractory metal oxide[s] containing alumina, silica,
zirconia, titania, ceria”) and “material composition B”
(e.g., an “aluminosilicate molecular sieve [with] one of the
crystal framework types FAU, MFI, MOR, BEA”). Id., col.
BASF CORPORATION   v. JOHNSON MATTHEY INC.                5



2, lines 29–58. And it includes various examples of how
catalyst layers are prepared and how they perform under
practical engine conditions in comparison to the prior art.
Id., col. 13, line 54 through col. 19, line 14.
                             B
    In its opening brief on claim construction in this case,
BASF urged that the “composition . . . effective to cata-
lyze” phrases have a plain and ordinary meaning, so it
proposed simply using those phrases followed by the
qualification, “as understood in the art of exhaust sys-
tems.” J.A. 105. BASF also argued that the phrases are
not indefinite, contrary to the contention Johnson had
stated in advance of claim construction.
     Johnson responded that the phrases are indefinite,
because the “effective to catalyze” language used to identi-
fy the claim compositions is functional, and there are no
“objective boundaries on (1) what amount of effectiveness
is required, or (2) how to measure the effectiveness.” J.A.
380; see id. at 379–93. Johnson’s expert, Dr. William S.
Epling, filed a declaration in support. He stated that
there are effectively a “limitless number” of materials that
can catalyze ammonia oxidation or SCR reactions. J.A.
953, 955. Regarding SCR catalysts, he stated that the
materials listed in the ’185 patent specification were
known in the art to be effective catalysts for SCR of NOx.
Regarding AMOx catalysts, Dr. Epling listed various
materials he considered to be known in the art as effective
catalysts. He also stated that “objective standards,” such
as “percent conversion,” exist in the field of catalysis to
test and quantify catalytic function. J.A. 957. Neverthe-
less, he concluded that the claims here are indefinite
because the materials given in the specification are not an
“exhaustive list” and the patent “does not define the level
of function required to be considered ‘effective’” or the
“particular conditions” under which a material would
have to be effective. J.A. 954, 956; see J.A. 954–58. For
6                        BASF CORP.   v. JOHNSON MATTHEY INC.



those reasons, Dr. Epling opined, a person of ordinary
skill in the art “would not be able to determine with
reasonable certainty the boundary of which materials are
included within” the claims. J.A. 955; see J.A. 953–56.
    In reply, BASF argued that the claims are not indefi-
nite. It rejected Johnson’s core contentions that the
patent had to specify a “level” of effectiveness and the
“conditions” under which that level would be achieved.
Rather, BASF argued, based on ordinary meaning and the
considerable information in the specification about exam-
ples and testing conditions, “the disputed ‘material com-
position’ limitations” (those at issue here) “should be
construed to encompass all compositions known to those
of skill in the art that perform the recited SCR or AMOx
functions,” J.A. 1165, to whatever degree and under
whatever conditions would be viewed by a relevant skilled
artisan as making the material an SCR or AMOx catalyst,
J.A. 1162–68.
      BASF attached a declaration from an expert, Dr.
Mark Crocker, that responded to Dr. Epling. He stated
that a person of ordinary skill in the art would under-
stand “composition . . . effective to catalyze” in the context
of the ’185 patent to mean a “composition capable of
catalyzing” the reaction in question “in a catalyst system
for treating engine exhaust gas.” J.A. 1318. And he
stated that such a “person would be reasonably certain
about the scope of the claims from reviewing the ’185
patent claims and the specification (including the exem-
plary compositions, exemplary measurements of catalytic
performance, and exemplary evaluation conditions).” Id.
“[I]t was well-known to persons of ordinary skill in the art
that the catalytic performance would vary based on
environmental considerations, such as temperature,” he
said, and “known variance in catalytic performance would
not have created confusion among ones of ordinary skill in
the art as to whether a material composition was an
‘effective’ SCR catalyst or an ‘effective’ AMOx catalyst,
BASF CORPORATION   v. JOHNSON MATTHEY INC.                 7



i.e., a material composition that could achieve or perform
catalysis.” J.A. 1319. He concluded: “The exemplary
evaluation conditions identified in the specification are
fairly standard and well-known. From these disclosures,
one of ordinary skill in the art would be reasonably cer-
tain as to what falls within the scope of the material
compositions in the disputed terms.” J.A. 1322.
     When Johnson filed a sur-reply brief, it attached a
short additional declaration from Dr. Epling. He asserted
that a person of ordinary skill in the art would not inter-
pret the claims of the ’185 patent to include “any material
that possessed any degree of effectiveness” to catalyze
AMOx or SCR of NOx and that “scientists and engineers
in this field do not consider materials that display only a
minimal level of SCR or ammonia oxidation function to be
a material effective to reduce NOx or to oxidize ammo-
nia.” J.A. 1410.
                             C
    The district court agreed with Johnson.      The court
reasoned as follows:
   Each claim fails to limit the “material composition
   A” or the “material composition B” to any specific
   materials. Rather than explicitly defining the
   material compositions, the claims utilize function-
   al language, specifically “effective,” to purportedly
   define them. In other words, the claims recite a
   performance property the composition must dis-
   play, rather than its actual composition. Moreo-
   ver, none of the claims recite a minimum level of
   function needed to meet this “effective” limitation
   nor a particular measurement method to deter-
   mine whether a composition is “effective” enough
   to fall within the claims. Without such infor-
   mation, a person of ordinary skill in the art could
   not determine which materials are within the
8                       BASF CORP.   v. JOHNSON MATTHEY INC.



    “material composition A” or “material composition
    B” limitation, and which are not.
J.A. 5. In a footnote, the court, quoting Dr. Epling, added
that “‘a practically limitless number of materials’ exist
that would ‘catalyze SCR of NOx, even within the normal
operating conditions of an exhaust aftertreatment sys-
tem,’ indicating that the claims, as written, fail to suffi-
ciently identify the material compositions.” Id. at 5 n.10
(quoting J.A. 955).
                             II
    The Supreme Court in Nautilus, Inc. v. Biosig In-
struments, Inc. held that a patent claim is indefinite if,
when “read in light of the specification delineating the
patent, and the prosecution history, [the claim] fail[s] to
inform, with reasonable certainty, those skilled in the art
about the scope of the invention.” 134 S. Ct. 2120, 2124
(2014). “Reasonable certainty” does not require “absolute
or mathematical precision.” Biosig Instruments, Inc. v.
Nautilus, Inc., 783 F.3d 1374, 1381 (Fed. Cir. 2015)
(internal quotation marks omitted). Johnson had the
burden of proving indefiniteness by clear and convincing
evidence. Id. at 1377.
    We review a determination of indefiniteness de novo.
Id. Determinations about governing legal standards and
about intrinsic evidence are reviewed de novo, and any
factual findings about extrinsic evidence relevant to the
question, such as evidence about knowledge of those
skilled in the art, are reviewed for clear error. Id. at
1377–78, 1382; see Teva Pharm. USA, Inc. v. Sandoz, Inc.,
135 S. Ct. 831, 841–42 (2015); UltimatePointer, L.L.C. v.
Nintendo Co., Ltd., 816 F.3d 816, 826 (Fed. Cir. 2016);
Teva Pharms. USA, Inc. v. Sandoz, Inc., 789 F.3d 1335,
1342 (Fed. Cir. 2015).
BASF CORPORATION   v. JOHNSON MATTHEY INC.                 9



                             A
    Under Nautilus, the question presented here is this:
would the “composition . . . effective to catalyze” language,
understood in light of the rest of the patent and the
knowledge of the ordinary skilled artisan, have given a
person of ordinary skill in the art a reasonably certain
understanding of what compositions are covered? The
district court’s reasoning supplies no basis to answer that
question in Johnson’s favor.
    The court first described the functional character of
the claim language. But the Nautilus standard of “rea-
sonable certainty” does not exclude claim language that
identifies a product by what it does. Nothing inherent in
the standard of “reasonable certainty” precludes a rele-
vant skilled artisan from understanding with reasonable
certainty what compositions perform a particular func-
tion. Not surprisingly, we have long held that nothing in
the law precludes, for indefiniteness, “defining a particu-
lar claim term by its function.” Hill-Rom Servs., Inc. v.
Stryker Corp., 755 F.3d 1367, 1374–75 (Fed. Cir. 2014);
see Cox Commc’ns, Inc. v. Sprint Commc’n Co. LP, 838
F.3d 1224, 1232 (Fed. Cir. 2016) (explaining that claims
“are not per se indefinite merely because they contain
functional language”), cert. denied, 137 S. Ct. 2267 (2017);
Microprocessor Enhancement Corp. v. Tex. Instruments
Inc., 520 F.3d 1367, 1375 (Fed. Cir. 2008) (explaining that
“apparatus claims are not necessarily indefinite for using
functional language”); In re Swinehart, 439 F.2d 210, 212
(CCPA 1971) (ruling that “there is nothing intrinsically
wrong with the use of such a technique in drafting patent
claims”). What is needed is a context-specific inquiry into
whether particular functional language actually provides
the required reasonable certainty.
     The district court next stated that the claims do not
“recite a minimum level of function needed to meet this
‘effective’ limitation nor a particular measurement meth-
10                       BASF CORP.   v. JOHNSON MATTHEY INC.



od to determine whether a composition is ‘effective’
enough to fall within the claims.” J.A. 5. By itself, that
observation merely describes two things not expressly
stated in the claims. But “an inventor need not explain
every detail because a patent is read by those of skill in
the art.” Wellman, Inc. v. Eastman Chem. Co., 642 F.3d
1355, 1367 (Fed. Cir. 2011). The mere observation of
information not “recited” does not answer the question
whether a person of ordinary skill in the art would need to
be given the level and measurement information to un-
derstand, with reasonable certainty, whether a composi-
tion is “effective to catalyze” the SCR (of NOx) or AMOx
reactions.
    Indeed, the district court did not treat the mere ob-
servation about information not “recited” as itself answer-
ing the question. The court immediately went on to
declare that “[w]ithout such information, a person of
ordinary skill in the art could not determine which mate-
rials are within the ‘material composition A’ or ‘material
composition B’ limitation, and which are not.” J.A. 5.
That sentence is the crucial sentence in the district court’s
analysis.
    The problem with that sentence, however, is that it is
entirely unsupported, whether by reference to the specifi-
cation or other intrinsic evidence or by reference to ex-
trinsic evidence.    Such support was central to our
determination that indefiniteness of certain physical-
property claims was proved in cases such as Dow Chemi-
cal Co. v. Nova Chemicals Corp. (Canada), 803 F.3d 620,
633–35 (Fed. Cir.), rehr’g denied, 809 F.3d 1223 (Fed. Cir.
2015); Teva, 789 F.3d at 1342–45; Halliburton Energy
Services, Inc. v. M-I LLC, 514 F.3d 1244, 1252–54 (Fed.
Cir. 2008); and Honeywell International, Inc. v. Interna-
tional Trade Commission, 341 F.3d 1332, 1340–42 (Fed.
Cir. 2003). The district court’s analysis in the present
case lacks such support for its conclusion about what a
BASF CORPORATION   v. JOHNSON MATTHEY INC.                 11



relevant skilled artisan could determine without more
information than the patent here provides.
    The district court’s analysis does not consider that the
specification makes clear that it is the arrangement of the
SCR and AMOx catalysts, rather than the selection of
particular catalysts, that purportedly renders the inven-
tions claimed in the ’185 patent a patentable advance over
the prior art. As a result, the claims and specification let
the public know that any known SCR and AMOx catalysts
can be used as long as they play their claimed role in the
claimed architecture. The district court’s analysis also
does not address the significance of the facts that both the
claims and specification provide exemplary material
compositions that are “effective” to catalyze the SCR of
NOx and the oxidation of ammonia, disclose the chemical
reactions that define the “SCR function” and “NH3 oxida-
tion function,” ’185 patent, col. 5, lines 33–49, and illus-
trate through figures, tables, and accompanying
descriptions how the purportedly novel arrangement of
the catalysts results in improved percent conversion of
ammonia and improved nitrogen selectivity, see id., cols.
13–19.
    The district court’s footnote adds nothing helpful to
Johnson. It credits Dr. Epling’s assertion that “a practi-
cally limitless number of materials” could catalyze SCR of
NOx, and it treats that scope as “indicating that the
claims, as written, fail to sufficiently identify the material
compositions.” J.A. 5 n.5. But the inference of indefinite-
ness simply from the scope finding is legally incorrect:
“breadth is not indefiniteness.” SmithKline Beecham
Corp. v. Apotex Corp., 403 F.3d 1331, 1341 (Fed. Cir.
2005) (internal brackets omitted).
                              B
    In this court, Johnson has supplemented the district
court’s reasoning. But we do not find persuasive support
for the necessary conclusion that a relevant skilled arti-
12                      BASF CORP.   v. JOHNSON MATTHEY INC.



san would lack reasonable certainty as to what composi-
tions are “effective to catalyze” the reactions at issue—or,
equivalently, what compositions are SCR catalysts or
AMOx catalysts—in the context of this patent.
     Johnson suggests that certain intrinsic evidence
shows that this particular patent departs from a relevant
skilled artisan’s general understanding of what consti-
tutes an SCR or AMOx catalyst and, instead, requires
some minimal level of catalysis but fails to identify that
level. In particular, Johnson points to language describ-
ing a certain catalyst (Q) that the patent calls an “SCR-
only” catalyst, which under some conditions had a low
“NH3 percent conversion” and under other conditions had
a conversion percent higher than another catalyst (P) that
the patent calls an “SCR+AMOx catalyst.” ’185 patent,
col. 18, lines 3–9 & tbl. 2; id. figure 10. From that mate-
rial in the specification, and another passage referring to
“AMOx-only parts,” id., col. 18, line 1, Johnson infers that
this patent requires, but fails to specify, a distinctive,
patent-specific set of criteria for what constitutes an SCR
catalyst or an AMOx catalyst.
    Johnson has read too much into the specification.
Contrary to Johnson’s essential contention in this specifi-
cation-based argument, it strongly appears that the
language on which Johnson relies—“SCR-only,” “AMOx-
only,” and “SCR+AMOx”—does not refer to whether a
composition can produce only one or the other or both of
the catalytic reactions. Instead, the language refers to
the catalysts being located in different “zones” on the
substrate. For example, the specification describes the
“upstream zone compris[ing] an SCR catalyst washcoat
layer disposed on the carrier and the downstream zone
compris[ing] an undercoat layer containing an NH3 oxida-
tion component disposed on the carrier and an SCR
catalyst washcoat layer disposed on at least a portion of
the undercoat layer.” Id., col. 11, lines 15–20. The speci-
fication goes on to describe the upstream portion as the
BASF CORPORATION   v. JOHNSON MATTHEY INC.                13



“SCR zone” and the downstream portion as the “AMOx
zone.” Id., col. 11, lines 29–30, 49–50. The labels, as
merely structure-describing shorthands, do not carry the
implications that Johnson urges. Johnson has given us no
persuasive reason to conclude that a relevant skilled
artisan, reading the claims in light of the specification,
would conclude that this patent departs from such an
artisan’s general understanding and instead adopts a
special, patent-specific standard without identifying what
it is.
    The intrinsic evidence in this case makes clear that
the asserted advance over the prior art is in the partly-
dual-layer arrangement to create a two-phase operation
for performing the identified conversion processes, not in
the choices of materials to perform each of the required
catalytic processes. It is in this context that the question
of the certainty or uncertainty experienced by a relevant
skilled artisan in understanding the claims, read in light
of the specification, is presented. And it is in this context
that the relevant skilled artisan would be informed by the
specification’s numerous examples of qualifying composi-
tions A and B, disclosure of the stoichiometric reactions,
and equating of the “composition . . . effective to catalyze”
phrases with familiar terms such as “SCR catalyst” and
“AMOx catalyst.”
    The extrinsic evidence does not show that a person of
ordinary skill in the art would lack reasonable certainty
as to what compositions would qualify as an SCR or
AMOx catalyst in this context. To the contrary, the
record here could not support a finding of lack of such
reasonable certainty.
    Both parties’ experts agreed that materials capable of
performing the claimed reactions were known in the art
at the time of the invention. The experts also agreed that
objective tests to determine the effectiveness of the cata-
lysts in question, e.g., percent conversion, were available
14                      BASF CORP.   v. JOHNSON MATTHEY INC.



and well known at the time. BASF’s expert declared, with
explanation, that “one of ordinary skill in the art would be
reasonably certain as to what falls within the scope of the
material compositions in the disputed terms.” J.A. 1322.
And Johnson’s expert, Dr. Epling, did not provide sub-
stantial evidence to the contrary.
    Dr. Epling’s declaration describes his own work with
“SCR catalysts,” speaks of “ammonia oxidation catalysts,”
gives examples of materials that can catalyze under
which conditions, and identifies materials that would “to
some measurable extent[] catalyze the oxidation of am-
monia if used to treat diesel engine emissions.” J.A. 951,
953. In those ways, Dr. Epling’s declaration implicitly
confirms that the terms at issue are ones whose scope is
understood with reasonable certainty by relevant skilled
artisans. Dr. Epling’s declaration also states: “In my
experience, scientists and engineers in th[e] field do not
consider materials that display only a minimal level of
SCR or ammonia oxidation function to be a material
effective to reduce NOx or to oxidize ammonia.” J.A.
1410. That assertion does not contradict the position of
BASF and its expert, which was that relevant skilled
artisans would reasonably understand what level (under
what conditions) qualify a composition as a claimed
catalyst. In fact, Dr. Epling’s assertion tends to confirm
the existence of just that understanding.
    This record, we conclude, does not contain intrinsic or
extrinsic evidence that would support a judgment of
indefiniteness.
                            III
    For the foregoing reasons we reverse the district
court’s judgment that the claims of the ’185 patent are
indefinite. We remand for further proceedings.
            REVERSED AND REMANDED
