What is the test for determining whether a contract is ambiguous or uncertain?

'When a contract is in any of its terms or provisions ambiguous or uncertain, 'it is primarily the duty of the trial court to construe it after a full opportunity afforded all the parties in the case to produce evidence of the facts, circumstances, and conditions surrounding its execution and the conduct of the parties relative thereto.'' Walsh v. Walsh, 18 Cal.2d 439, 443, 116 P.2d 62, 65.

Sections 1856 and 1860, Code of Civil Procedure, provide the basic rules of construction in this respect.