Enharmonic intervals · Inversion and consonance · Music theory
Two intervals are enharmonic when they sound the same and are written differently. C–F♯ (augmented 4th) and C–G♭ (diminished 5th) both span six half steps;…
Enharmonic intervals
Two intervals are enharmonic when they sound the same and are written differently. C–F♯ (augmented 4th) and C–G♭ (diminished 5th) both span six half steps; C–E♭ (minor 3rd) and C–D♯ (augmented 2nd), three.
As with notes, the spelling decides the name: in A harmonic minor, F–G♯ is an augmented 2nd, never a minor 3rd.
The tritone
The augmented 4th and the diminished 5th split the octave exactly in two: the only interval that still measures the same after inversion. Its six half steps make it very unstable, and medieval theory avoided it warily; hence the later nickname diabolus in musica.
In tonal music that tension is exactly what is put to use. According to Piston, the diminished 5th tends to close into a 3rd, with both voices moving a 2nd inward, and its inversion, the augmented 4th, tends to open into a 6th. It is the engine of the cadence.