Through past practices in administering the labor contract of a particular plant, the values and standards of the parties are revealed, along with the expectations of employees and managers on particular matters. The paper first defines the nature of a practice: conduct having clarity and consistency, longevity and repetition, and general acceptability. The author also reviews the varied content of practices and the kind of proof required in an arbitration hearing to prove the existence of an alleged practice. The functions of past practice are then reviewed: (1) clarifying ambiguous language in the labor contract, (2) implementing general contract language, (3) modifying or amending apparently unambiguous language, and (4) setting a separate enforceable employment condition. The paper's concluding section considers the duration and termination of a practice. It advises that if either side objects to the continuance of a past practice at the time a contract is being negotiated, then the practice will continue to be binding only if it is written into the agreement. Also, a practice must be related to the continuance of the conditions from which it arose. When those conditions substantially change, the practice may be subject to termination.