There is one more useful interval which we have not yet discussed - the perfect unison. This means one note played by two different instruments together, or simultaneously in two different positions on the one instrument.
An example of this would be the note E played as an open string on the 1st string and also at the 5th fret on the 2nd string. Unisons can be useful in many styles of guitar playing, particularly when using slides or bends.
Here are some examples of harmonic intervals (two notes played together). Write the name of each interval above or below the notation and play them on the guitar to be sure you are familiar with the sound of each interval.
For more practice with intervals, write and play the indicated intervals above each note in the following example.