Key signature key signatures showing sharps or flats on both octaves for notes which fall within the staff. An accidental is an exception to the key signature, applying only to the measure and octave in which it appears. Although a key signature may be written using any combination of sharp and flat symbols, the most common series of thirteen homogeneous key signatures—ranging from six flats to six sharps, with each successive flat or sharp placed on the note a perfect fifth below or above, respectively, the previous one—is assumed in much of this article. A piece scored using a single diatonic key
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;