Patent ambiguity prevents extrinsic evidence from supplying meaning, leaving inherently indeterminate contractual terms unresolved. A patent ambiguity is an inherent uncertainty in an instrument's language that cannot be resolved by ordinary rules of legal construction or by extrinsic explanatory evidence; where terms are either capable of a definite external meaning (such as recognized mercantile usages) effect may be given by adopting that meaning, but where terms are incapable of any certain interpretation they remain indeterminate and cannot be supplied by evidence of intention.