The minor second on 8-string guitar

A minor second is one semitone, from C to D♭. Inverted, with the lower note moved up an octave, it becomes the major seventh.

Its short name is m2, and it is also called semitone or half step. Counted in letters, it spans two letter names, which is why it is written C to D♭ rather than with another name for the same pitch.

On 8-string guitar, the minor second is four frets back and three frets back when you cross to the next string, and one fret 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 D♭.

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

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

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

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.