The defendant in Berwick & Smith Co. v. Salem Press, Inc. thought that "copy" meant a two-volume set (so the printing and binding actually cost twice what the defendant expected). The defendant testified that he was unaware of the trade usage. The courts correctly held that this lack of knowledge was irrelevant.
Where there is trade usage relevant to the interpretation of a term in a particular contract, the presumption is to interpret that term in accord with the trade usage even if one of the parties was unaware of the trade usage."
See the flowchart on the next screen for a summary of the rules on interpretation so far.