An octave is a natural division of sound, and a natural phenomenon which is discovered in every civilization which develops a musical culture.

Let us number the keys 1 to 12. Each key one after the other produces a sound which keeps rising by one note over the previous key. The first 12 keys represent (in piano) what we call the mandra saptak (low octave), the next 12 keys represent the madhya saptak (middle octave) and the last 12 keys represent the tār saptak (high octave).