R.I. Gen. Laws § 6-26.1-4: Interest.

A credit card lender may charge and collect interest under a credit card plan on outstanding unpaid indebtedness in the borrower’s account under the plan at any daily, weekly, monthly, annual, or other periodic percentage rate or rates that the agreement governing the plan provides or as established in the manner provided in the agreement governing the plan. If the agreement governing the plan so provides, the outstanding unpaid indebtedness may include the amount of any interest, interest fees and charges, and other charges outstanding. Interest may be calculated using an average daily balance; two-cycle, average daily balance; adjusted balance; previous balance method; or using any other balance computation method provided for in the agreement governing the plan. Credit card transactions may be included in the outstanding unpaid indebtedness as of any time as may be specified in the agreement governing the plan. Periodic billing cycles may be established in any manner and shall have the duration that may be specified in the agreement governing the plan.