Understanding Guitar Intervals

Intervals are the distance between 2 notes. On a guitar, we can measure these in terms of frets, where one fret is equal to one semitone. There are 8 primary intervals: Unison (or Root note), 2nd, 3rd, 4th, 5th, 6th, 7th, and 8th (or Octave). These intervals span 12 frets or semitones before repeating across octaves.

The 2nd, 3rd, 6th, and 7th intervals consist of both minor and major qualities, which are denoted with a lowercase 'm' and an uppercase 'M' respectively. The 4th and 5th are considered 'perfect' intervals denoted with a 'P'. The interval between the perfect 4th and perfect 5th (the tritone) is considered an augmented 4th or a diminished 5th depending on context, but we will use 'T' and refer to it simply as the tritone here. The unison and octave are also considered 'perfect' intervals as well, but in our application we refer to them as the root note, marked with an 'R'.

When using intervals, the distance remains the same (for example, a minor second is always one semitone above the root), but the actual pitch notes they represent can be moved anywhere on the fretboard. This means we can change which note we consider the root, but always refer to the note one fret above it as a minor second ('m2').

This level of abstraction allows us to think and play music more generally, especially on fretted instruments which are very shape-based. It also allows us to easily change keys of songs or play more complex chord extensions without having to memorize millions of isolated shapes. You just need to know which chord tones you want, and understanding where they are relative to the root will allow you to play exactly what you hear in your head.