Overdubbing (the process of making an overdub, or overdubs) is a technique used by recording studios to add a supplementary recorded sound to a previously recorded performance.
Tracking (or "laying the basic tracks") of the rhythm section (usually including drums) to a song, then following with overdubs (solo instruments, such as keyboards or guitar, then finally vocals), has been the standard technique for recording popular music since the early 1960s.
Overdubs can be made for a variety of reasons. One of the most obvious is for convenience; for example, if a bass guitarist is temporarily unavailable, the recording can be made and the bass track added later. Similarly, if only one or two guitarists are available, but a song calls for multiple guitar parts, a guitarist can play both lead and rhythm guitar (For example, System of a Down's "Chop Suey!" or Deep Purple's "Smoke on the Water", where it would have been impossible for the guitarists to play the solos on electric guitars and riffs on rhythm guitars simultaneously). Singers who also play an instrument find overdubbing a convenience, since it allows them to focus on one role at a time.
Many vocalists use overdubbing to effectively sing harmony with themselves, as did Michael Jackson, ABBA, Patti Page, Prince, Harry Nilsson, Laura Nyro, The Carpenters, Robert Plant, John Frusciante, Radiohead, Brian Wilson, Elliott Smith, Emitt Rhodes, James Hetfield and George Harrison (who credited himself as the "George O'Hara-Smith Singers"). Overdubbing can be done multiple times: the members of Queen overdubbed their voices numerous times to create the chorus effect for "Bohemian Rhapsody", while Electric Counterpoint by Steve Reich features a solo guitarist recording 10 guitars and 2 electric bass parts and then playing the final 11th guitar part live.
Singers in particular have also used the practice to perfect a recorded performance over several takes. Singer Dusty Springfield is widely reported to have painstakingly assembled individual phrases over time in order to record the ultimate performance. Overdubbing is also used to solidify a weak singer; doubletracking allows a singer with poor intonation to sound more in tune. (The opposite of this is often used with sampled instruments; detuning the sample slightly can make the sound more lifelike.)
Overdubbing has sometimes been viewed negatively, when it is seen as being used to artificially enhance the musical skills of an artist or group, such as with studio-recorded inserts to live recordings, or backing tracks created by session musicians instead of the credited performers. The early records of The Monkees were made by groups of studio musicians pre-recording songs (often in a different studio, and some before the band was even formed), which were later overdubbed with the Monkees' vocals. While the songs became hits, many critics cried foul, and Michael Nesmith in particular disliked having to "duplicate someone else's records" for their television show. No cheating was intended in the practice (with the emphasis on the TV program, and with the four members not an experienced group, it was felt that using studio 'ringers' would be more efficient), and numerous other singers and groups had studio help, but this didn't save the Monkees from critical and public scorn.
Perhaps the earliest commercial issue of recordings with overdubs was by RCA Victor in the late 1920s, not long after the introduction of electric microphones into the recording studio. Recordings by the late Enrico Caruso still sold well, so RCA took some of his early records made with only piano accompaniment, added a studio orchestra, and reissued the recordings.
Sidney Bechet made a pair of famous overdubbed sides in 1941, "The Sheik of Araby" and "Blues of Bechet". Multi-instrumentalist Bechet recorded on six different instruments; each version had to be recorded onto a new master disc along with the preceding performance, with consequent loss of audio quality. The novelty was issued as "Sidney Bechet's One Man Band". The American Federation of Musicians protested the recording, putting an end to experiments with commercial overdubbing in the United States for years.
The invention of magnetic tape opened up new possibilities for overdubbing, particularly with the development of multitrack recording with sel-sync. The first commercially released overdubbed recording made on multitrack magnetic tape was by guitarist Les Paul, whose 1947 record "Lover (When You're Near Me)", featured eight different electric guitar parts. His later work would be seminal in the popularization of multitrack recording.
The Buddy Holly track "Words of Love" featured Holly overdubbing his own guitar and voice, inspired, as seen in the Buddy Holly Story, by Les Paul.
Peter Ustinov performed multiple voices on "Mock Mozart", in a recording produced by George Martin. Abbey Road Studios had no multitrack recorders at the time, so a pair of mono machines was used. Martin used the same process later for a Peter Sellers comedy record, this time using stereo machines and panning.