The perfect fourth on 8-string guitar

A perfect fourth is five semitones, from C to F. Inverted, with the lower note moved up an octave, it becomes the perfect fifth.

Its short name is P4. Counted in letters, it spans four letter names, which is why it is written C to F rather than with another name for the same pitch.

On 8-string guitar, the perfect fourth is the same fret and one fret up when you cross to the next string, and five frets up along one string.

One pair of 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.

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 is what the interval's name counts. What you hear is a distance of five semitones.

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

32 notes are marked across eight strings.

This diagram marks both notes of the perfect fourth, C and F, everywhere they fall on 8-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.