The music of the gamelan is based on one of two different tuning systems or scales, slendro and pelog. Certain instruments, such as the bonang and gender, have two sets of instruments; one is tuned in slendro and the other in pelog. This pair of instruments is often positioned at right angles to each other so that musicians can switch from one to another during a performance. Compositions do not mix the two tuning systems.

   The slendro tuning divides the octave into five fairly equal parts and sounds like the pentatonic ("black keys') scale of Western music. The basic spacing of intervals from lowest to highest is 2 3 2 2 3 (where 2 is a whole step and 3 is approximately a minor third). Here are approximate equivalents in the Western scale:

   The seven intervals of the pelog tuning vary in size, and is roughly similar to the minor scale. The spacing between the intervals is 1 2 3 1 1 2 2 (where 1 is about a half-step, 2 is about a whole step, and 3 is approximately a minor third). It is rare to have all seven notes of the pelog scale used in a piece. Rather, five-note modal scales derived from the seven are used. Here are approximate equivalents in the Western scale:

   No two gamelan sets have the same exact tuning. There are no Javanese standard forms of these two tuning systems. One of the reasons is that historically, Javanese tradition dictated that the design and tuning of old and sacred gamelan sets could not be copied - each ensemble has an individual identity. The sense that it is improper to copy old gamelan tunings is still strong in Indonesia. Instead, each set can be appreciated for its unique sound quality. Also, musicians will use different interval structures to maximize the distinctive character of the gamelan instruments in a set--whether they be sad, joyous, or majestic, for example.