When a contract is contained in a series of letters, the entire series of letters collectively constitutes the documentary evidence of the contract. According to the principles of the Indian Evidence Act, 1872, particularly Section 91, when the terms of a contract have been reduced to the form of a document or a series of documents, no evidence shall be given in proof of the terms of such contract except the document itself or secondary evidence where permissible. If the contract is formed through correspondence, all the letters that form part of and evidence the terms of the contract must be produced or accounted for (if the original is lost, secondary evidence may be given). Proving only one letter, or only the concluding one, or only the disputed portion in isolation would not give a complete and accurate picture of the entire contractual agreement.