Simple intervals
Intervals within an octave, as opposed to compound intervals such as the ninth or tenth that exceed an octave.
What it means
Simple intervals are all pitch distances that do not exceed an octave, ranging from the unison to the octave itself: seconds, thirds, fourths, fifths, sixths, and sevenths belong here, as do the boundary cases of the unison and the octave. Any larger pitch distance, by contrast, can be described as a combination of one or more octaves plus a simple interval.
Musical use
The division into simple and compound intervals makes harmonic and melodic analysis easier, since larger pitch distances such as the ninth or the twelfth can be traced directly back to their corresponding simple interval, here the second and the fifth respectively. In practice, simple intervals are therefore often taught as the basic building blocks from which more extended intervals are derived through octave extension.
Simple intervals should be distinguished from compound intervals such as the ninth, the tenth, the eleventh, or the twelfth, each of which comprises an octave plus a simple interval. Sonically and functionally, a simple interval and its compound counterpart usually behave similarly, since octave extension generally does not change the basic harmonic character.
Performance practice
Practise recognising simple intervals within a single octave on the piano or your own instrument before tackling compound intervals, since the latter are easier to identify once the corresponding simple interval is reliably recognised. When hearing a large, unfamiliar pitch distance, mentally reduce it by one or more octaves first to determine the underlying simple interval.