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18-1. By Outside Agency
If a ball at rest is moved by an outside agency, there is no penalty and the ball must be replaced.

Note: It is a question of fact whether a ball has been moved by an outside agency. In order to apply this Rule, it must be known or virtually certain that an outside agency has moved the ball. In the absence of such knowledge or certainty, the player must play the ball as it lies or, if the ball is not found, proceed under Rule 27-1.

(Player’s ball at rest moved by another ball — see Rule 18-5.)

18-2. By Player, Partner, Caddie or Equipment
• a. General
When a player’s ball is in play, if:

  (i) the player, his partner or either of their caddies lifts or moves it, touches it purposely (except with a club in the act of addressing it) or causes it to move except as permitted by a Rule, or
  (ii) equipment of the player or his partner causes the ball to move, the player incurs a penalty of one stroke. If the ball is moved, it must be replaced unless the movement of the ball occurs after the player has begun the stroke or the backward movement of the club for the stroke and the stroke is made.

Under the Rules there is no penalty if a player accidentally causes his ball to move in the following circumstances:

• In searching for a ball in a hazard covered by loose impediments or sand, for a ball in an abnormal ground condition or for a ball believed to be in water in a water hazard — Rule 12-1
• In repairing a hole plug or ball mark — Rule 16-1c
• In measuring — Rule 18-6
• In lifting a ball under a Rule — Rule 20-1
• In placing or replacing a ball under a Rule — Rule 20-3a
• In removing a loose impediment on the putting green — Rule 23-1
• In removing movable obstructions — Rule 24-1

• b. Ball Moving After Address
If a player’s ball in play moves after he has addressed it (other than as a result of a stroke), the player is deemed to have moved the ball and incurs a penalty of one stroke. The ball must be replaced unless the movement of the ball occurs after the player has begun the stroke or the backward movement of the club for the stroke and the stroke is made.

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