The rule excluding parol evidence to vary a written contract obtains only between the parties thereto, or their successors in interest, so that defendant, in an action for a broker's commissions for procuring a purchaser by R of land from defendant, may show that the contract between R. and defendant, even though in form one of sale, was merely an option and net acted on by R. Brown v. Wisner, 51 Wash. 509, 99 P. 581.