Learning how to play “double stops” – two notes at a time (on different strings) – is a skill every guitar player should learn. The most obvious place to start is to learn two of the high strings. And when you understand the way the strings interact together you quickly realize that the two strings you should start on are the G and B strings.
Because most of your double stops will consist of 3rds – either major or minor 3rds – a combination of the 1st and 3rd notes in the scale. As you will soon see, you can move up through a major scale by simply playing a major or minor 3rd on each note. And it is easiest to play these combinations on the G and B strings.
As we saw in a previous post, adjacent notes are 5 semitones apart. But between the G and B strings, adjacent notes are only 4 semitones apart.. And since a major third consists of notes 4 semitones apart, several of your fret combinations will be on adjacent strings.
Let me explain this further…
Major and Minor 3rds
For the most part, for songs written in a major key you can play note combinations in 3rds that match the chords belonging to a major scale: Major – Minor – Minor – Major – Major – Minor – Minor – Major. For example, for the G scale the note combinations are G-B, A-C, B-D, C-E, D-F#, E-G, F#-A, G-B. In terms of semitones these combinations are 4 semitones apart (major), 3 (minor), 3 (minor), 4 (major), 4 (major) 3 (minor) 3 (minor), 4 (Major).
On the G and B strings a major 3rd is played at adjacent frets (because they are 4 semitones apart). A minor 3rd is played by dropping your finger one fret on the B string. This is about as easy as it gets on the guitar.