The Tritone & Altered Qualities
Discover the 'Devil in Music' and how perfect intervals expand or contract.
The Tritone
The Tritone is an interval of exactly 6 semitones (three whole tones). Because it splits the octave exactly in half, it is symmetrical, meaning its inversion is also a tritone. In medieval times, it was called 'Diabolus in Musica' (the devil in music) due to its highly unstable, dissonant sound that demands resolution.
Augmented & Diminished Intervals
When we alter a Perfect or Major interval by adding or subtracting a half-step, we create Augmented or Diminished qualities:
Created when a Perfect interval (or a Minor interval) is shrunk by one semitone. For example, a Perfect 5th (7 semitones) shrunk by a half-step becomes a Diminished 5th (6 semitones).
Created when a Perfect interval (or a Major interval) is expanded by one semitone. For example, a Perfect 4th (5 semitones) expanded by a half-step becomes an Augmented 4th (6 semitones).
Summary
The Tritone sounds the same whether written as an Augmented 4th or a Diminished 5th (same sound, different names). Remember: Shrunk Perfects become Diminished, Expanded Perfects/Majors become Augmented.