protect the consumer (or the bank, as it may be) from wild fluctuations of the prime rate. While these accounts are harder to budget for, they can theoretically be a little less expensive since the bank does not have to accept the risk of fluctuation of the market (since the prime rate follows inflation rates, which affect the profitability of loans). A fixed rate can be better for consumers who have fixed incomes or need control over their payments budgets. These rates can be varied upon depending upon the policies of different organisations. Grace period Many banks provide an exception to their normal method of calculating interest, in which no interest is charged on an ending statement balance that is paid by the due date. Banks have various rules. In some cases the account must be paid off for two months in a row to obtain the discount. If the required amount is not paid, then the normal interest rate calculation method is still used. This allows cardholders to use credit cards for the convenience of the payment method (to have one invoice payable with one check per month rather than many separate cash or check transactions), which allows them to keep money invested at a return until it must be moved to pay the balance, and allows them to keep the float on the money borrowed during each month. The bank, in effect, is marketing the convenience of the payment method (to receive fees and possible new lending income, when the cardholder does not pay), as well as the loans themselves. Promotional interest rates Many banks offer very low interest, often 0%, for a certain number of statement cycles on certain sub-balances ranging from the entire balance to purchases or balance transfers (used to pay off other accounts), or only for buying certain merchandise in stores owned or contracted with by the lender. Such "zero interest" credit cards allow participating retailers to generate more sales by encouraging consumers to make more purchases on credit. Additionally, the bank gets a chance to increase income by having more money lent out, and possibly an extra marketing transaction payment, either from the payee or sales side of the business, for contributing to the sale (in some cases as much as the entire interest payment, charged to the payee instead of the cardholder). These offers are often complex, requiring the cardholder to work