The grace period is the window between your statement date and payment due date during which you can pay off your full balance without incurring interest on that period's purchases, commonly around three weeks in length though the exact number of days varies by bank. This grace period generally only applies if you paid your previous statement balance in full - carrying forward any balance from a prior month typically means new purchases start accruing interest immediately, without the benefit of a grace period, until the full balance is cleared again.
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