The parol evidence rule is not, properly speaking, a rule of evidence but a rule of substantive law. It deals with what the terms of a contract in law are, and not merely with the evidence by which it is to be proved.1 Accordingly, the law of the forum does not apply to this rule.2 So if a suit is brought on a contract of indorsement, the question whether an oral agreement limiting liability to the amount collected on certain collateral assigned over for convenience in collection, is admissible as a defense is determined by the law controlling the contract and not by the law of the forum.3