when you sign up for a credit card its purchase apr and other aprs will be shown in your credit card agreement 
those rates can change over time for several reasons for example the credit card issuer is allowed to change the rate if it gives you 45 days notice and states a reason for doing so the reason might be a late payment that you made or a recent drop in your credit score any new purchases you make with the card more than 14 days after you ve received the advance notice will be subject to the new rate 3however credit card companies are prohibited from increasing interest rates on new transactions during the first year after an account is opened 3many cards also impose a penalty or default apr that is triggered if a payment is late or you have exceeded the card s credit limit the penalty apr always applies to future purchases after such an event but it can also be applied to the existing balance if a payment is more than 60 days late 3your purchase apr can also change if your card charges a variable rather than a fixed rate as noted above no apr is totally fixed since it can be changed by the card issuer with 45 days notice with a variable rate card however the issuer is free to change it quarterly or monthly based on the movement of a particular index such as the prime rate and isn t required to give you advance notice 4 your new rate would be the prime rate or other benchmark index plus some set percentage your credit card agreement should indicate how much the apr can change over time 5