Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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.