The notes that you play on the guitar are measured in pitch. Pitch corresponds with how high or low a note sounds. The distance in pitch between two notes is called an interval.

The smallest interval is a semitone. A semitone is the distance in pitch between two adjacent frets on a guitar. If you play fret 5 on any string of the guitar, then play fret 6 on the same string, you will hear the difference. An interval of 2 semitones (two frets) can also be called a tone. An interval of 12 semitones (12 frets) is called an octave.

1 fret = a semitone
2 frets = a tone
12 frets = an octave

On the chromatic scale, a semitone is the distance between two adjacent notes (A and A#). If you ascend 12 semitones in the chromatic scale you end up on note you started on, but the note is an octave higher in pitch.

No. of semitones Interval name
1 Minor second
2 Major second
3 Minor third
4 Major third
5 Perfect fourth
6 Augmented fourth/Diminished fifth
7 Perfect fifth
8 Minor sixth
9 Major sixth
10 Minor seventh
11 Major seventh
12 Perfect eight/Perfect octave