The tritone on 7-string guitar

A tritone is six semitones, from C to F♯. Inverted, with the lower note moved up an octave, it becomes the tritone.

Its short name is TT, and it is also called augmented fourth or diminished fifth. Its name counts whole tones, three of them. Written C to F♯ it spans four letter names and is an augmented fourth. Written C to G♭ it spans five letter names and is a diminished fifth. Both are the same six semitones.

On 7-string guitar, the tritone is one fret up and two frets up when you cross to the next string, and six frets up along one string.

One pair of neighboring strings is the odd one out, because the gap between those two strings is different from the rest. Most players learn that exception as a rule of its own, and it moves every shape that crosses the gap, including the ones that skip a string.

On a treble staff, the two notes are written C and F♯.

The staff shows the two notes one after the other, then together. C to F♯ spans four letter names, which makes it an augmented fourth. The name tritone counts its three whole tones instead. What you hear is a distance of six semitones.

A keyboard makes this distance easy to see, where a fretboard hides it. The upper note is six keys along from the lower one, counting black and white keys alike.

19 notes are marked across seven strings.

This diagram marks both notes of the tritone, C and F♯, everywhere they fall on 7-string guitar. You can find every pair shown above somewhere on it.

C–F♯, D♭–G, D–G♯, E♭–A, E–A♯, F–B, F♯–B♯, G–C♯, A♭–D, A–D♯, B♭–E, B–E♯

The shapes above do not change with the key, which is the reason to learn an interval as a shape. Only the note names change, and this table gives them for every starting note.

Three chord types contain a tritone: Diminished, Half-diminished, Diminished 7th.

These are the chords with a tritone above their root. Some of them spell that note under another name, but it is the same distance and the same shape on the neck. A chord is built by stacking intervals on a root, and a triad is the simplest case, with two of them.