Laws of Cricket 1947 Code (Fourth Edition) - Law 23
The ball shall be bowled from each wicket alternately in Overs of either 8 or 6 balls according to the agreed conditions of play. When the agreed number have been bowled and is has become clear to the Umpire at the Bowler's wicket that both sides have ceased to regard the ball as in play, the Umpire shall call "Over" in a distinct manner before leaving the wicket. Neither a "No Ball" nor a "Wide Ball" shall be reckoned as one of the "Over".
Note on Law 23
1. In the United Kingdom the "over" shall be 6 balls, unless an agreement to the contrary has been made.
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