A rock band or pop band is a small musical ensemble which performs rock music, pop music or a related genre. The four-piece band is the most common configuration in rock and pop music. Before the development of the electronic keyboard, the configuration was typically two guitarists (a lead guitarist and a rhythm guitarist, with one of them singing lead vocals), a bassist, and a drummer (e.g. the Beatles, KISS, Metallica). Another common formation is a vocalist who does not play an instrument, electric guitarist, bass guitarist, and a drummer (e.g. the Who, the Monkees, Led Zeppelin, Queen, and U2). Instrumentally, these bands can be considered as trios.