R.I. Gen. Laws § 28-9-27

(1) An arbitrator shall have the authority to consider the existence of a past practice that may exist between the parties to a collective bargaining agreement only under the following circumstances: the collective bargaining agreement does not contain an express provision that is the subject of the grievance, or

(2) the collective bargaining agreement contains a provision that is unclear and ambiguous, or

(3) the collective bargaining agreement contains a provision which has been mutually agreed upon by the parties that preserves existing past practices for the duration of the collective bargaining agreement.

(4) A party claiming the existence of a past practice shall be required to prove by clear and convincing evidence that the practice; is unequivocal;

(5) has been clearly enunciated and acted upon;

(6) is readily ascertainable;

(7) has been in existence for a substantial period of time;

(8) has been accepted by representatives of the parties who possess the actual authority to accept the practice.

(9) A past practice that may exist between the parties to a collective bargaining agreement may not override any contrary provision of an existing collective bargaining agreement, statute or ordinance.

(10) A past practice that may exist between the parties to a collective bargaining agreement may not override any contrary provision of any written rule, regulation, or policy that has been promulgated, adopted, and published pursuant to either the Administrative Procedures Act or promulgated and published by the appropriate governing entity in a city or town.

(11) Any party to a collective bargaining agreement may provide written notice to the other party that it no longer intends to be bound by a past practice unless the collective bargaining agreement contains a provision which has been mutually agreed upon by the parties that preserves existing past practices for the duration of the collective bargaining agreement. This notification must describe the past practice and set forth the effective date of the termination of the practice. Neither party is obligated to follow the practice thirty (30) days following this notification.