All production software in XP is built by two programmers, sitting side by side, at the same machine. This practice ensures that all production code is reviewed by at least one other programmer, and results in better design, better testing, and better code.
It may seem inefficient to have two programmers doing “one programmer’s job”, but the reverse is true. Research into pair programming shows that pairing produces better code in about the same time as programmers working singly. That’s right: two heads really are better than one!
Some programmers object to pair programming without ever trying it. It does take some practice to do well, and you need to do it well for a few weeks to see the results. Ninety percent of programmers who learn pair programming prefer it, so we highly recommend it to all teams.
Pairing, in addition to providing better code and tests, also serves to communicate knowledge throughout the team. As pairs switch, everyone gets the benefits of everyone’s specialized knowledge. Programmers learn, their skills improve, they become more valuable to the team and to the company. Pairing, even on its own outside of XP, is a big win for everyone.
(Source: Ron Jeffries)