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An XP practice where two programmers work alongside each other, trying to get a task accomplished. Two minds at one problem.
What does this bring about:
1) It brings up productivity if the pair knows what it is upto. A chatty pair can cause more damage to the project than getting done
2 ) It keeps each person honest. If you dont know something its apparent in the fifth minute.
3) It helps keep the quality of the code. Since the pair is helping code.
4) This is where design happens.
Agile Practitioners Guide 