In music notation, a flat is a symbol resembling a lower-case 'b' which indicates that a certain note should be lowered a half step. If used in the key signature, it means that note should be lowered a half-step whenever it is encountered; if used as an accidental, it means that only that note should be changed.

The half-step decrement applies whether it changes the note to a "white key" or not. For instance, A-flat is the same as G-sharp, but F-flat is the same as E.

A flat looks something like this, when used in the key signature and as an accidental with a half note:

----| /----------------------------------------------------
    |/    b                                                
----/--------------b O-------------------------------------
   /|               |                                      
 |  |  |   b        |                                      
  \ |  |