The first music written specifically for four instruments was by Joseph Haydn, who composed during the classical period in the 18th century. The music of this period emphasized the intimacy and individualism of a small group of instruments, as opposed to large orchestras. Mozart and Beethoven are also famous for their quartet compositions.
Two hundred years later, Jazz music developed in the United States. At first, Jazz music was played by large groups, but in the 1940s, the bebop movement popularized smaller groups, and the quartet became a standard feature of this genre. In the 50s and 60s, rock and roll, which was heavily influenced by jazz, became popular. The standard rock group, such as the Beatles or Radiohead, is a quartet.
A classical quartet is made up of four stringed instruments: two violins, a viola and a cello. The violins are the smallest instruments in this group, and have the highest pitch. They are referred to as "first violin" and "second violin," and they play different musical parts. Sometimes, a piano or wind instrument plays one of the violin parts. Violas looks like violins, but they are bigger, with a lower range. Cellos are the largest instrument in a quartet, and they provide the lowest musical part. Together, these four different parts balance and harmonize with each other.
Jazz quartets can be composed of many different instruments, but they usually include one instrument from each of the following categories: a stringed instrument, such as a guitar, fiddle or piano; a wind instrument such as a saxophone, clarinet or trumpet; a bass instrument, which can be an electric or acoustic bass; and a percussion instrument, usually a drum kit.
Rock quartets, usually called rock bands, are composed of a singer, a guitar player, a bass player and a drummer. Variations on this recipe are common, however: the singer could play the guitar and another musician could play the keyboard, for example. No matter what the combination, however, rock quartets almost always contain a bass and drums. Called the rhythm section, these two instruments provide the steady beat that is a defining feature of rock music.