Time signature
The number in the top of the time signature tells a player how many of a certain kind of note there are in each measure. The number in the bottom of the time signature tells what kind of note is used a certain number of times in each measure. The number on the bottom of the time signature can be any power of 2 that is greater than or equal to 2. So, 64 could be a number that is put in the bottom of the time signature, but 65 could not be one.