
369 S.E.2d 113 (1988)
MARSH REALTY COMPANY
v.
2420 ROSWELL AVENUE.
No. 8726SC1244.
Court of Appeals of North Carolina.
June 21, 1988.
*115 Smith, Helms, Mulliss & Moore, by Rolly L. Chambers, Charlotte, for plaintiff-appellee.
Ruff, Bond, Cobb, Wade & McNair by Moses Luski, Charlotte, for defendant-appellant.
ARNOLD, Judge.
Defendant contends that the trial court improperly interpreted the lease and erred in granting plaintiff's motion for summary judgment. We agree.
The provision of the lease governing rental payments attempts to provide a formula for determining rent for years twenty through seventy-five of the lease. Under the formula, the percentage increase in the Cost-of-Living Index is to be applied to the "base rental period" in order to determine rent for years twenty to thirty of the lease. The lease states that the "base rental period" is "hereinafter defined" but the definition is never given. Accordingly, there is a question whether the amount of rent for the "base rental period" is $5,000.00 (the rent from 1965 to 1975) or $6,000.00 (the rent from 1975 to 1985).
Plaintiff asserts that the "base rental period" figure is $6,000.00 while defendant argues that the "base rental period" figure is $5,000.00. Under plaintiff's calculations, it would be entitled to $20,751.60 per year from 1985 to 1995. Under defendant's calculations, plaintiff would receive only $17,293.00 per year from 1985 to 1995.
When the language of a contract is plain and unambiguous, its construction is a matter of law for the court. DeTorre v. Shell Oil Co., 84 N.C.App. 501, 353 S.E.2d 269 (1987). If the language used in a contract is ambiguous and the parties' intention is unclear, the interpretation of the contract is for the jury. Parker Marking Sys., Inc. v. Diagraph-Bradley Indus., Inc., 80 N.C.App. 177, 341 S.E.2d 92, disc. rev. denied, 317 N.C. 336, 346 S.E.2d 502 (1986).
The formula set out in the lease to determine annual rent from 1985 to 1995 is ambiguous. The lease fails to define "base rental period" which is crucial to the application of the formula. While the fairest result may have been reached, the trial court erred in granting summary judgment because an issue of fact remains concerning the amount of annual rent for 1985 to 1995.
Plaintiff cross-assigns error to the trial court's failure to consider affidavits of persons involved in negotiating the terms of the lease. We need not address plaintiff's argument because the affidavits are not properly before this Court as a part of the record on appeal.
Vacated and remanded.
ORR and GREENE, JJ., concur.
