There are 12 different key signatures (24 including the relative minors). The circle of 5ths is a useful way to remember how many sharps and flats each of these key signatures has. It involves starting in the key of C and moving down by 5ths until you eventually get back to C. Moving down a 5th or up a 4th is the same thing. This and other complementary intervals are discussed in the relative minor section.

Enharmonic

In the circle of 5ths there are some keys that have two names. These keys have the ability to be interpreted in two different ways as they are enharmonic. You will notice that Gb has 6 flats. This key could also be named F# which would have 6 sharps. Gb calls a B note Cb and F# calls F E#. A single note on its own could be called either by its flat or sharp name (eg. C# or Db) but a note in the context of a key must be named according to the correct key as listed here, i.e. in the key of E it would be C# but in the key of Ab it would be Db.