A heptatonic scale is a musical scale with seven pitches per octave. Among the most famous of these are the major scale, C D E F G A B C; the melodic minor scale, C D E♭ F G A B C ascending, C B♭ A♭ G F E♭ D C descending; the harmonic minor scale, C D E♭ F G A♭ B C; and a scale variously known as the Byzantine, Hungarian,gypsy, or Egyptian scale, C D E♭ F♯ G A♭ B C. South Indian (Carnatic music) classical theory postulates seventy-two melakarta, seven-tone scale types, whereas Hindustani classical music postulates twelve or ten (depending on the theorist) seven-tone scale types collectively called thaat.

The term diatonic scale refers to a pitch collection and does not imply any particular tonal center, mode or note of special emphasis^{[citation needed]}. It is in this respect different from the term major scale, which does imply a tonal center.

In traditional classical theory the melodic minor scale has two forms, as noted above, an ascending form and a descending form. Although each of these forms of itself comprises seven pitches, together they comprise nine, which might seem to call into question the scale’s status as a heptatonic scale. In certain twentieth-century music, however, it became common systematically to use the ascending form for both ascending and descending passages. Such a use has been notably ascribed to the works of Bela Bartok and to bop and post-bop jazz practice. The traditional descending form of the melodic minor scale is equivalent to the natural minor scale in both pitch collection (which is diatonic) and tonal center.