Whether you are interested in spicing up a lead or just adding an occasional short riff to your accompaniment playing, you will want to investigate playing harmonic intervals, also known as double stops. All you have to do to play double stops is to play both notes of the interval at the same time!
Here are the shapes for major and minor thirds:
The shape that is shown between strings 6 and 5 will be used on all combinations of strings except the second and third string combination. Because of the way the guitar is tuned, the shape of any interval that involves these two strings will need to be condensed.
It’s a good idea to practice the intervals of the scale over all six strings, but you will want to spend more time on the higher strings, since you will probably be using them the most in your playing. You can practice moving thirds up and down the neck on the first set of strings by playing the intro to Van Morrison’s “Brown Eyed Girl.” Start on the 7th fret of the second string and the 8th fret of the first string, which will produce a major third. Use your ear to find the notes from that point.
Next, learn to play the intervals of minor and major sixths as double stops. Using the same approach that you did for thirds, you will learn them as they occur in the major scale and then experiment with them in your playing. Here are the shapes for the sixths:
The following notation shows how the sixths can be played in the key of C, using scale pattern #1:
In the above example, the upper line of notes is the C scale. After you learn to play the scale this way, try playing melodies in sixths. Use a simple melody that you are familiar with and harmonize all or part of it in sixths.
In addition to playing melodies, try adding interest to your accompaniments by using sixths to move from chord to chord. For example, instead of using a bass line to connect the chords, use the same notes an octave higher and harmonize with a sixth, as in the following example:
Now that you have put some attention on harmonic intervals (double stops), you may have started to notice how often they occur in the music you love. You will hear them in every genre and will discover them as signature licks from artists as varied as Chuck Berry, Muddy Waters, Jimi Hendrix and Bill Monroe.