Laws of Cricket 1947 Code (Fourth Edition) - Law 30
If the ball, not having been called "Wide" or "No Ball", pass the Striker without touching his bat or person, and any runs be obtained, the Umpire shall call or signal "Bye"; but if the ball touch any part of the Striker's dress or person except his hand holding the bat, and any run be obtained, the Umpire shall call or signal "Leg Bye"; such runs to be scored "Byes" and "Leg Byes" respectively.
Notes on Law 30
1. As an experiment, the following will apply: Leg byes shall be scored only if, in the opinion of the umpire, the striker
1. has attempted to play the ball with his bat, or
2. has tried to avoid being hit by the ball.
In the case of deflection by the striker's person, other than in (a) or (b) above, the umpire shall call "dead ball" as soon as one run has been completed, or the ball has reached the boundary and such runs shall be disallowed.
2. The umpire signals "Bye" by raising an open hand above the head, and "Leg Bye" by touching a raised knee with the hand.
Reproduction of the Laws of Cricket is by kind permission of Marylebone Cricket Club