Reference : compound and simple intervals
Simple intervals are not greater than an octave, while compound intervals are greater than an octave. Ninths, tenths, elevenths, and thirteenths are examples of compound intervals. Octaves, thirds, and fifths are simple intervals.
Each compound interval is related to a simple interval. If you subtract 7 from a compound interval, you get the corresponding simple interval. For example, 9 - 7 = 2, so a ninth is related to a second. A compound interval has the same quality as the simple interval to which it is related. Here are some examples of compound intervals and their corresponding simple intervals:
See Reading Music in the Tutorials section.
