173. Admission of oral evidence.

In general the parol evidence rule applies to written charters, thus excluding oral evidence of an intent to contract on different terms1. But oral evidence is admissible if words used are ambiguous2 or used in a technical sense3, or to supplement them by proving an independent collateral undertaking4. Since, moreover, it is well known that different trades and different ports have usages of their own5 which regulate the supply6, loading7 or discharging8 of the cargo, or by which the extent of the duties9, rights10, or liabilities11 of the parties are to be determined, the parties must be taken, in