The function of the tanpura
The function of the tanpura in Indian music is twofold. Firstly, the tanpura tuned to the tonic creates a drone, and it is to this drone that the notes of the raga relate. Indian classical music is modal in form, consequently giving the drone a fundamentally prominent function.
The second basic function involves the consonant and dissonant tones produced by the raga, the former suggesting satisfaction, peace and rest, and the latter tension, excitement and unrest. Furthermore, the tension or unrest caused by dissonant notes must always be resolved into rest. That rest we find in the tonic or the drone.