In his 1960 paper, Professor Mittenthal defined a past practice as “the understood and accepted way of doing things over an extended period of time.” In this article, he notes that, in the intervening years, the parties have taken steps to express their obligations in writing, and many employers have negotiated highly restrictive arbitrability clauses. While still a valuable interpretive aid, today, the legal model of arbitration proposed by Braden predominates. The author discusses past practice as a binding condition of employment supported by “mutual agreement,” and the scope, duration and termination thereof; and past practice as a means of clarifying ambiguity.