Indian Evidence Act, 1872
93. Exclusion of evidence to explain or amend ambiguous document
When the language used in a document is, on its face, ambiguous or defective, evidence may not be given of facts which would show its meaning or supply its defects.
(a) A agrees, in writing, to sell a horse to B for "Rs. 1,000 or Rs. 1,500".
Evidence cannot be given to show which price was to be given.
(b) A deed contains blanks. Evidence cannot be given of facts which would show how they were meant to be filled.