The minor third on 8-string guitar

A minor third is three semitones, from C to E♭. Inverted, with the lower note moved up an octave, it becomes the major sixth.

Its short name is m3. Counted in letters, it spans three letter names, which is why it is written C to E♭ rather than with another name for the same pitch.

On 8-string guitar, the minor third is two frets back and one fret back when you cross to the next string, and three 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 E♭.

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

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

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

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.

Six chord types contain a minor third: Minor, Diminished, Minor 7th, Half-diminished, Diminished 7th, Minor-major 7th.

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