Gong ensembles are also a common musical heritage of Island Southeast Asia. The casting of gong instruments are believed to have originated from the Bronze Age cultures of Mainland Southeast Asia. It spread to Austronesian islands initially through trade as prestige goods. However, Mainland Asian gongs were never used in ensembles. The innovation of using gong sets is uniquely Austronesian. Gong ensembles are found in western Malayo-Polynesian groups, though they never penetrated much further east. There are roughly two gong ensemble traditions among Austronesians, which also produced gongs in ancient times.