Metallophones have been used in music for hundreds of years. There are several different types used in Balinese and Javanesenese gamelan ensembles, including the gendér and saron. These instruments have a single row of bars, and are often tuned to a pentatonic scale. The glockenspiel and vibraphone are also metallophones - these have two rows of bars and are tuned to the chromatic scale.
In music of the 20th century and beyond, the word metallophone is sometimes applied specifically to a single row of metal bars suspended over a resonator box and tuned to the diatonic scale. Such instruments are often used in schools, and Carl Orff used them in several of his pieces, including his pedagogical Schulwerk.