Laws of Cricket 1947 Code (Fourth Edition) - Law 39
The Striker is out "Leg before wicket" - If with any part of his person except his hand, which is in a straight line between wicket and wicket, even though the point of impact be above the level of the bails, he intercept a ball which has not first touched his bat or hand, and which, in the opinion of the Umpire, shall have, or would have, pitched on a straight line from the Bowler's wicket to the Striker's wicket, or shall have pitched on the off-side of the Striker's wicket, provided always that the ball would have hit the wicket.
Note on Law 39
1. The word "hand" used in this Law should be interpreted as the hand holding the bat.
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