3rds and 6th intervals on the guitar are some of my favourite things to use to create riffs or just give a bit more interest to some lead parts. Some great examples of this already include Van Morrison's Brown Eyed Girl, John Mayer's Belief Cat Stevens' Peace Train and many more.
As mentioned before, these are great for creating riffs, putting in solos, or even just for chord progressions or a light 2nd guitar part, but how do we use them?
In any given major key, starting from the root, 3rds will follow the same pattern for chords
i.e. 1) Major 2) minor 3) minor 4) major 5) major 6) minor 7) diminished
Since we are only using 3rds and not the full chord, the 7th chord (the diminished) will just be a minor 3rd.
To see how this will look on the different strings of a guitar, check out these diagrams.
It's very important to remember that the shapes used on the G-B strings are different than all the other strings.
The pattern for the 6ths is slightly different than the 3rds. With 6ths we use:
1) Major 2) major 3) minor 4) major 5) major 6) minor 7) minor
Again, please check out these diagrams for a more visual representation.
It's also important to note that the shapes between E-D strings and A-G strings are different to the shapes on D-B strings and G-E strings.