Pelog is one of the essential tuning systems used in gamelan instruments that has a heptatonic scale. The other, older scale commonly used is called slendro. Pelog has seven notes, but many gamelan ensembles only have keys for five of the pitches. Even in ensembles that have all seven notes, many pieces only use a subset of five notes, sometimes the additional 4th tone is also used in a piece like western accidentals.