The minor seventh on 8-string guitar

A minor seventh is ten semitones, from C to B♭. Inverted, with the lower note moved up an octave, it becomes the major second.

Its short name is m7. Counted in letters, it spans seven letter names, which is why it is written C to B♭ rather than with another name for the same pitch.

On 8-string guitar, the minor seventh is five frets up and six frets up when you cross to the next string, and ten 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 B♭.

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

A keyboard makes this distance easy to see, where a fretboard hides it. The upper note is ten 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 minor seventh, C and B♭, everywhere they fall on 8-string guitar. You can find every pair shown above somewhere on it.

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

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 minor seventh: Dominant 7th, Minor 7th, Half-diminished.

These are the chords with a minor seventh above their root. A chord is built by stacking intervals on a root, and a triad is the simplest case, with two of them.