The octave on 5-string bass

An octave is twelve semitones, from C to C. Inverted, with the lower note moved up an octave, it becomes the unison.

Its short name is P8. Counted in letters, it spans eight letter names.

On 5-string bass, the octave is seven frets up when you cross to the next string, and twelve frets up along one string.

Every pair of neighbouring strings gives the same shape, because the strings of this tuning are evenly spaced. Once you have learned the shape, it works anywhere on the neck.

On a bass staff, the two notes are written C and C.

The staff shows the two notes one after the other, then together. C to C spans eight letter names, which is what the interval's name counts. What you hear is a distance of twelve semitones.

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

Nine notes are marked across five strings.

This diagram marks both notes of the octave, C and C, everywhere they fall on 5-string bass. You can find every pair shown above somewhere on it.

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

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.