Note that a bank not allowing overdrafts (e.g. by returning checks NSF if the account has insufficient funds for them to clear) does not guarantee that an account will never end up with a negative balance. In particular, you might deposit a check which is later returned (as NSF, or as being fraudulent) and not have enough money in your account to cover the check at the point when it is returned -- hence Patrick's point that checking accounts require the bank to extend credit even if the only thing you ever do is deposit checks.