Usage of trade; extrinsic evidence in contract disputes

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"When the words of a written instrument are used in their ordinary meaning, their construction presents a question solely of law.  But words are used sometimes in a peculiar meaning.  Then extrinsic evidence may be necessary to determine the meaning of words appearing in the document.  This is true where technical words or phrases not commonly understood are employed, or extrinsic evidence may be necessary to establish a usage of trade or locality which attaches provisions not expressed in the language of the instrument."