thirddiminished

Thirddiminished refers to the concept of a diminished third in music theory. An interval is diminished when its pitch distance is one semitone smaller than the standard minor or perfect form. A third is the diatonic distance of three letter names, such as C to E or A to C. A diminished third thus has the same letter-name distance as a third but a pitch distance of two semitones, one semitone smaller than a minor third. Because of this, a thirddiminished interval is enharmonically equivalent to a major second, though it is spelled as a third apart.

In practice, diminished thirds are largely theoretical and are encountered mainly in discussions of interval spelling, voice leading, or in certain chromatic contexts where precise diatonic spelling matters. They occur when the upper note is lowered by a semitone relative to a normal minor third while preserving the third-letter-name relationship (for example, A to Cb, where C is lowered to Cb). In equal temperament tuning, the pitches coincide with a major second, but the spelling signals a different harmonic or melodic function.

Thirddiminished is distinct from diminished triads or diminished seventh chords, which concern chordal structures rather than a single interval. It is one of several interval qualities that illustrate the complexity of interval naming in tonal and atonal theory.

Related topics include diminished intervals, major seconds, enharmonic spellings, and interval quality.