The unison on 8-string guitar

A unison is the same pitch, from C to C. Inverted, with the lower note moved up an octave, it becomes the octave.

Its short name is P1, and it is also called perfect prime. Counted in letters, it spans one letter name.

On 8-string guitar, the unison is five frets back and four frets back when you cross to the next string, and the same fret 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 C.

The staff shows the two notes one after the other, then together. C to C spans one letter name, which is what the interval's name counts. What you hear is the same pitch twice.

A unison uses one key, because it is a distance of nothing. It only becomes interesting on an instrument that can play the same pitch in more than one place, and a keyboard cannot do that.

16 notes are marked across eight strings.

This diagram marks every C on 8-string guitar. A unison is any two of these played together, which is why a unison has a shape on the neck at all.