A marimba is a non-transposing percussion instrument consisting of a set of wooden, or synthetic material for cheaper models, bars arranged similar to a piano keyboard but struck usually with rubber or yarn-wrapped rubber mallets. Performers typically use two or more mallets in each hand, secured by special grip devices, in order to play chords. Suspended underneath the bars are pipe-like resonators, today usually made from aluminum tubing, but formerly hollowed out gourds or carved wood, that amplify their sound. The keyboards cover about four octaves for models used in schools and five octaves for professional models. The sound is a more resonant and lower-pitched than the xylophone. Marimbas are today used in a wide range of music genres, both popular and classical, and even in some marching bands.