An interval is the distance between any two notes.
The distance from a note to any other can be measured and named.
To do this refer to the major scale of the lowest note.
Then number each note of this scale.
To identify the interval between ‘C’ and ‘A’. The lowest note being ‘C’ refer, to the C major scale.
The distance between ‘C’ and ‘A’ is a sixth.
However, the ‘A’ could be flat (Ab) or sharp (A#). The interval would still be a sixth, but there would be a variation in its description.
The table below shows the name of each interval.
m = minor
M = major
Aug = augmented
Dim = diminished
P = perfect
The distance between ‘C’ and ‘Ab’ is a minor sixth and the distance to A# is an augmented sixth.
Try these intervals on your guitar.