Key signature sometimes called "standard key signatures". Other scales are written either with a standard key signature and use accidental as required, or with a nonstandard key signature. Examples of the latter include the E (right hand), and F and G (left hand) used for the E diminished (E octatonic) scale in Bartók's "Crossed Hands" (no. 99, vol. 4, "Mikrokosmos"), or the B, E and F used for the D Phrygian dominant scale in Frederic Rzewski's "God to a Hungry Child". The absence of a key signature does not always mean that the music is in the key of C major /
Theoretical key Theoretical key In music theory, a theoretical key or impossible key is a key whose key signature has at least one double-flat or double-sharp. Double-flats and double-sharps are often used as accidentals, but placing them in the key signature (in music that uses equal temperament) makes the music generally impractical to read. For example, the key of G major is a key of this type, because its corresponding key signature has an F (on its leading-tone). An equal-tempered scale of G major comprises the same pitches as the A major scale, so that the two scales sound exactly the same;