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. The smallest ensemble that is commonly used in rock music is the trio format. Two-member rock and pop bands (such as Steely Dan, The White Stripes and The Black Keys) are relatively rare, because of the difficulty in providing all of the musical elements which are part of the rock or pop sound (vocals, chords, bass lines, and percussion or drumming). In a hard rock or blues-rock band, or heavy metal rock group, a "power trio" format is often used, which consists of an electric guitar player, an electric bass guitar player and a drummer, and typically one or more of these musicians also sing (sometimes all three members will sing, e.g. Bee Gees or Alkaline Trio). Some well-known power trios with the guitarist on lead vocals are the Jimi Hendrix Experience, Stevie Ray Vaughan and Double Trouble, Nirvana, the Jam, ZZ Top, and Green Day, while power trios with the bass guitarist on lead vocals include Cream, Rush, The Police and Motörhead.