If a drummer made a mistake, instead of rerecording the entire drum part, the engineer might decide to only fix the part where the drummer made a mistake. Thanks to advancements in digital recording, it's actually relatively easy to punch in and punch out of a track, meaning rerecording just a particular section of a track seamlessly.
But it can get complicated. For instance, if you record a band playing close together, the singer's microphone could have picked up the drummer's mistake. When a mic picks up another sound, the sound is said to "bleed" or "leak" into the mic. Bartlett explains that this makes the recording harder to mix later and creates a "distant, muddy sound."
One solution to this type of problem is to simply record all of the parts separately. In the same way that Les Paul liked to play multiple parts himself, recording engineers can construct a song one instrument at a time. If an engineer decides to record instruments separately, experts recommend starting with the drums for a good rhythm track. You should then move on the bass guitar followed by lead guitar and keyboards [source: Schonbrun]. Each musician after the drummer can listen to the previous recordings on headphones. Recording the different instruments and vocals in isolation this way makes them much easier to combine later on. Another bonus is cost-cutting: Bartlett mentions that you can use one microphone instead of several, saving money.
In addition to the instrument isolation, isolated recording allows for enormous flexibility. For instance, Bartlett says that one musician can record in Los Angeles; he or she can then send the recording to another musician in Chicago, who adds another track. The process of adding a new track onto previously recorded tracks is called overdubbing. Often, even if the original song were recorded with the entire band, the engineer can decide to overdub another instrument to the song for a powerful effect, like swelling strings or brassy horns.
If you're recording a song with an instrumental or vocal solo, Bartlett recommends recording several takes of it -- each on its own track. That way, he says, the engineer can actually "pick and choose the best parts of each take." The composite, or comp, that results can therefore be the ideal take that never actually happened and is better than any of the takes that did.