Augmented fourth

What it means

The augmented fourth comprises six semitones; it is the perfect fourth extended by a semitone. Together with the diminished fifth it forms the tritone — both sound the same on tempered instruments, yet their spelling says different things about their origin and their continuation.

In the notation

The difference is read off from the degree count: F–B is a fourth with four degrees, F–C-flat a fifth with five. Which spelling is chosen depends on the harmonic context and on the resolution. The augmented fourth strives apart — the lower note does not fall, but the upper one rises while the lower stays put or moves downward; the diminished fifth draws together.

Musical use

In the major scale, the augmented fourth arises between the fourth and seventh degrees. In the Lydian mode it lies between the root and the fourth degree and shapes its open, wide sound directly — a raised fourth degree above the root is the hallmark of this mode.

In the cadence it appears as a component of the dominant seventh chord, when this stands in a position where the seventh and third are measured as a fourth; the resolution then leads apart. In altered sonorities and in secondary dominants it is likewise common.

Historically the tritone was regarded as something to be avoided in melodic movement; musica ficta served, among other things, to avoid it by raising or lowering a note.

In the Lydian mode it is the hallmark, because it stands there between the root and the fourth degree. In major it occurs between the fourth and seventh degrees, for instance F–B in C major, and resolves outward into the sixth in the dominant seventh chord. As a melodic leap it was avoided in strict counterpoint.

Performance practice

Watch the direction: the augmented fourth opens, the diminished fifth closes. Take the rising note close to its goal. And derive the interval, when singing, from a familiar context — as a fourth plus a semitone, or from the Lydian sound — rather than hitting it in isolation.