Utility Workers Union of America
AFL - CIO Local 127

Depending on what your collective bargaining agreement says, it may be that the way things are in your workplace—established “past practices”—are actually enforceable under the contract or in another legal forum. Arbitrators resolving contract disputes will sometimes rule that employees have the right to do something the way it’s always been done, even if there’s no explicit language saying so in the contract. For example, at night you may park in a close-by customer lot because that’s where everyone always parks, even though the contract doesn’t say anything about parking. Suppose one day your employer puts up a sign prohibiting parking there. The union may well have a grievance it can win that the “past practice” of parking in that lot can’t be changed unilaterally.
—Adapted from The Union Member's Complete Guide, by Michael Mauer