Existing practices sanctioned by use and acceptance, that are not specifically included in the collective bargaining agreement. Arbitrators use evidence of past practices to interpret ambiguous contract language. In addition, past practices can be enforced under the negotiated grievance procedure because they are considered part of the agreement. See, e.g., 5 FLRA No. 35 and 7 FLRA No. 125. Unilateral changes in past practices dealing with conditions of employment (see 24 FLRA No. 96, 27 FLRA No. 44, and 34 FLRA No. 104) can constitute unfair labor practices(ULP). See, e.g., 6 FLRA No. 127, 9 FLRA No. 11, and 21 FLRA No. 103. Indeed, it is a ULP to unilaterally change a practice that is at odds with the express terms of the agreement. See, in this connection, 36 FLRA No. 65, where FLRA said the following:
The fact that the negotiated agreement addressed the matter is not conclusive, if it is shown, in fact, that over a period of time the parties had engaged in a practice regarding the [matter] that differed from the contractual procedure. If this showing is made, and the practice satisfies the statutory requirements of section 7103(a)(14), it is a condition of employment that cannot be unilaterally altered. Letterkenny Army Depot, 34 FLRA 606, 610-11 (1990).
As the ALJ noted in 42 FLRA No. 7, "[t]o find that a condition of employment has been established by past practice . . . there must be a showing that the practice was consistently exercised for an extended period of time, with the agency`s knowledge and express or implied consent." Norfolk Naval Shipyard, 25 FLRA No. 19 [25 FLRA 277 at 286].