<prompt> Human: Every thread that involves discussion of balance transfers to a 0 interest card inevitably has a comment warning you to pay it all off or youll be charged all the deferred interest.The CARD Act of 2009 prohibits any credit card that advertises 0 APR or 0 interest from charging deferred interest. If you fail to pay these cards off in full by the end of the promotional period, you will only be charged interest on the balance going forward. Of course, you should pay it all off anyway, since that was the whole point of the balance transfer.Retail cards like Best Buy do charge deferred interest on purchases. Youll notice their language says things like NO INTEREST IF PAID IN FULL WITHIN 6 MONTHS. They never say 0.If the card advertises a 0 APR, they cannot charge you deferred interest.  <response>: Wow -- I legitimately did not realize this. I just logged into my Discover It account to check the terms on my 0 18 month balance transfer, and youre right -- it only accrues interest on the remaining balance after the promotional period. Good to know that if I have a slip-up during the pay back period that it wont go back to where it started. Thank you OP! 