First, there must be strong proof that the practice exists. A past practice cannot rest on weak or unreliable evidence, such as hearsay.

Second, both parties must be aware of the practice. Awareness may be inferred when the practice is longstanding, widespread and visible enough that the employer or union reasonably should have known about it.

Third, the practice must be mutually accepted if it is to modify or amend the collective bargaining agreement. The binding nature of past practice is not simply because it happened many times, but because it is a practice that is agreed by both parties to be the prescribed way of doing things. Both sides must have implicitly or explicitly acknowledged and agreed to the practice (consensual). However, some practices are mere coincidences that developed unintentionally, or they are choices within management discretion, meaning that they are not mutually agreed upon and thus cannot be enforced as past practices. Past practice should not overrule clear, unambiguous contract language.

Fourth, the practice must be clear, consistent and continuous. If the parties have handled the issue in materially different or inconsistent ways over time, there is no single accepted past practice that the parties agreed to or for an arbitrator to enforce.

Fifth, the practice is limited to the circumstances that gave rise to it. A past practice applies only to comparable situations. If the underlying conditions change, the practice is no longer in effect.

Once established, a past practice generally remains in effect until the parties negotiate it away or the circumstances supporting it change. If the practice is not addressed during collective bargaining, arbitrators usually infer that the parties intended it to continue. An objection to the past practice during negotiations does not automatically end the practice. It remains binding unless both parties agree to eliminate or modify it.