INTERPRETATION A MATTER OF LAW. The interpretation or construction of a con tract is for the court, that is, it is a matter of law. \Vhere the dispute is as to what was said in an oral contract it is a question of fact for the jury to determine. But when the agreement between the parties is proved it is for the court to determine its effect, and neither party can avoid the plain meaning of his words by claiming he did not intend what he said. So where the parties have written out their agreement in full, by a rule of evidence they are precluded from adding to or varying such written agreement by oral or parol evidence. This general rule of evidence applying to written contracts is subject to certain exceptions, or the rule when inter preted admits certain extrinsic evidence, and this ad missible evidence Anson classifies under three heads: "r. Proof of existence of document. Evidence as to the fact that there is a document purporting to be a contract, or part of a contract.
"2. Of fact of agreement. Evidence that the pro fessed contract is in truth what it professes to be. It may lack some element necessary to the formation of contract, or be subject to some parol condition upon which its existence as a contract depends.
`3. Of terms of contract. Evidence as to the terms of a contract. These may require illustration which ne cessitates some extrinsic evidence; or they may be ambiguous and then may in like manner be explained; or they may comprise, unexpressed, a usage the na ture and effect of which have to be proved.
"We are thus obliged to consider (1) evidence as to the existence of a document, (2) evidence that the document is a contract, (3) evidence as to its terms." (Anson on Contract, pp. 238-9.)