The octave on drop D guitar

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 drop D guitar, the octave is five frets up, seven frets up and eight frets up when you cross to the next string, and twelve frets up along one string.

The shape changes from one pair of strings to another, because the strings of this tuning are not evenly spaced. Instead of one shape to learn, there are three of them.

On a treble 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.

Twelve notes are marked across six strings.

This diagram marks both notes of the octave, C and C, everywhere they fall on drop D guitar. 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.