Science Fair Project Encyclopedia
A substitute in the sport of cricket is a replacement player that the umpires allow when a player has been injured or become ill after the nomination of the players at the start of the game. The rules for substitutes all appear in Law 2 of the Laws of cricket.
A substitute can act for the injured or ill player in the field, although he may not bowl, bat, or act as wicket-keeper or as captain. The substitute may also act as a runner when the injured or ill player is batting, but may not bat himself. A player may bat, bowl and field even if he has had a substitute for part of the game. If a player has a runner, he may be given out if either he or his runner transgresses the rules.
If a batsman gets injured or ill while batting, he may retire and resume his batting at a later stage in the innings at the fall of a wicket or when another batsman retires hurt. If he takes the umpire's permission to leave, he is declared 'Retired – not out'.
If a batsman retires without the umpire's permission, he may only resume his innings with the opposing captain's permission. If he does not resume his innings, he is given out. This often happens from time to time in friendly practice games, for instance English county sides versus University Centres of Cricketing Excellence. Only two batsmen have retired out in test match cricket. Both instances were in the same match, where the Sri Lankan batsmen, retired out to save the Bangladeshi opposition further embarrassment. Each of these two batsmen had already scored more runs than the entire Bangladeshi team in the previous innings, and it is arguable that they deliberately gave batting time to newer teammates. 
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