The term backline used to refer just to audio amplification equipment that stands behind the band on stage, including amplifiers for guitars, bass guitars and keyboards. In the US, backline has expanded in recent years to include the instruments that the musicians play, from guitars and bass guitars, to keyboards and organs, to drum kits and various percussion instruments such as guitar, bass and other percussion instruments. It is often used in this sense to talk generally about the equipment available to or needed by musicians. (e.g. "What is the backline at O'Malley's Pub? The band wants to know if they need to bring their own drums or amps.")
Backline equipment can be rented for touring or studio use. Many travelling musicians prefer not to carry their own backline across borders and continents for fear of damage or customs hassles, which makes renting backline equipment while on tour an attractive option.
Backline technicians look after, set up and maintain the backline equipment. Backline techs who travel with touring acts are also known as roadies.
PA equipment, which is generally placed in the "frontline" so as to reduce problems of acoustic feedback, is not part of the back line. In rock music's early days, the PA and the band's amplification were all set in a line, which conceptually grouped PA and instrument amplification together. This changed after the late 1960s, when the band's instrumentation was set behind the PA to create the modern audio stage set-up. It is from this stage set-up that modern monitoring techniques, as well as the concepts of frontline and backline, developed.