The card issuer charges interest only on the outstanding balance at the end of the previous billing cycle. (creditcardeducation.com)
However, the moment you let a month lapse without paying off your balance in full, you'll start paying interest on all the purchases you generated throughout that previous billing cycle. (creditcardeducation.com)
* APR = Annual Percentage Rate ** A finance charge will be imposed on credit purchases only if you elect not to pay the entire new balance shown on your monthly statement for the previous billing cycle within 28 days from the closing date of that statement. (frankenmuthcu.org)