Laws of Cricket 1947 Code (Fourth Edition) - Law 3
Before the toss for innings two umpires shall be appointed; one for each end to control the game as required by the Laws with absolute impartiality. No Umpire shall be changed during a match without the consent of both captains.
Note on Law 3
1. The umpires shall report themselves to the executive of the ground 30 minutes before the start of each day�s play.
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