What method is used to calculate the monthly finance charge for the first major credit card?

The monthly finance charge for a credit card is typically calculated using the average daily balance method or the adjusted balance method. In the average daily balance method, the issuer sums the daily balances throughout the billing cycle and divides by the number of days in the cycle, then multiplies by the monthly interest rate. The adjusted balance method, on the other hand, calculates the balance after payments and credits are applied, before applying the interest rate. The specific method used can vary by issuer and card agreement.