The gamelan orchestra, based on metallic percussion with winds and drums, is perhaps the form which is most readily identified as being distinctly "Javanese" by outsiders. In various forms, it is ubiquitous to Southeast Asia. In Java, the full gamelan also adds a bowed string instrument (the rebab, a name illustrative of Islamic influence) and voices. The rebab is one of the main melodic instruments of the ensemble (together with the xylophone "gendér") and is often played by the senior musician. Voices consist of male and female choruses, together with soloists; however, the voices are not usually featured in court gamelan (as opposed to wayang kulit, shadow puppet theatre) and are supposed to be heard discreetly in the middle of the orchestral sound. In these abstract pieces, the words are largely secondary to the music itself.
There are two scales in Javanese gamelan music, "slendro" (pentatonic) and "pelog" (heptatonic in full, but focussing on a pentatonic group). Tuning is not standard, rather each gamelan set will have a distinctive tuning. A complete gamelan consists of a pair of sets, one tuned in each of the scales and intended to be played together in many instances. Different gamelan sets have different sonorities, and are used for different pieces of music; many are very old, and used for only one specific piece. Musical forms are defined by the rhythmic cycles. These consist of major cycles subdivided by smaller cycles, each marked by the striking of successively smaller gongs. The melodic interplay takes place within this framework (technically called "colotomic structure"). There are also distinct melodic modes ("patet") within the division of scale.