Sixths
The interval of a sixth can be calculated by counting up six notes from the lower note, e.g.:
In scales of 6ths, the harmony notes are most commonly played a 6th below the scale note..
Here is the C scale in 6ths.
This gives the following pattern on the second and fourth strings.
Sixth: 2nd-4th String Pattern
Use the pick to play the lower note and the third finger* to play the higher note, as illustrated in the photo.
This pattern can be repeated on the first and third strings. The keynote will be the top note (in pitch) of the two starting notes.
*By using the 3rd finger, the 2nd finger is left free to play other notes if desired.
Sixths played on the 4th and 6th strings, and the 3rd and 5th strings will involve a different pattern, as such:
Sixth: 4th-6th, 3rd-5th String Pattern
