The outlined tritone

A line avoids outlining the tritone: when the tones at a phrase segment’s turning points — where it changes direction or comes to rest — stand a tritone apart, the ear hears the forbidden interval even though no single leap made it. The classic case is the fourth degree and the leading tone bounding one gesture: fa against mi. The repair is a turn short of the offending tone, or carrying the line past it so the boundary falls elsewhere.

The rule shapes lines wherever the fourth degree and the leading tone live close together — scale-wise counterpoint, cantus firmus writing, chant-style melody — and it is why so many strict-style lines turn exactly where they do. It survives into free styles as taste: a melodic boundary of a tritone still sounds unstable, which composers exploit deliberately when instability is wanted and avoid when it is not.

• Running a stepwise ascent from the fourth degree straight to the leading tone, the textbook outlined tritone.
• Turning the line at two tones a tritone apart, so the boundary interval is the diabolus even with steps between.
• Fixing a vertical tritone while leaving the melodic outline intact, repairing the chord but not the line.
• Banning every tritone that occurs inside a longer scale run, when the rule concerns the boundaries of the gesture, not its interior.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, mi contra fa — The dialogue’s name for the fault, and the turns that keep the line clear of it.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), The preliminary rules (the tritone) — The melodic tritone barred alongside the augmented leaps.