18-1. If an outside agency moves a ball at rest, there is no penalty; the player must replace the ball. In general, an outside agency is anything other than a player, caddie, a ball in play on the hole or a player’s equipment. Wind and water aren’t considered outside agencies.

18-2. If a player, his partner or either of their caddies moves, picks up or purposely touches the ball (except with the club during address), or the player’s or partner’s equipment causes the ball to move, there’s a one-stroke penalty and the ball must be replaced.

There’s also no penalty if the ball is moved while the player repairs a plugged hole or pitch mark on the green, measures its distance from the hole, is marking and lifting the ball, moving a loose impediment on the green or moving an obstruction.

If the ball moves after the player takes his address, he’s penalized one stroke penalty and the ball must be replaced (unless he strikes the ball after it moves).

18-3. In match play, there’s no penalty if the player, opponent, caddie or his equipment causes the ball to move while searching for it. The ball must be replaced, however. If the opponent or his caddie intentionally moves the ball, other than during a search, the opponent is penalized one stroke.

18-4. In stroke play, if a player’s competitor, his caddie or equipment moves the ball (intentionally or not), there’s no penalty and the ball must be replaced.

18-5. A ball in play and at rest must be replaced, at no penalty, if another ball hits it.

18-6. There’s no penalty if a ball or ball marker is moved while measuring distance on the green; the ball must be replaced.

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

Except as permitted by the Rules, when a player’s ball is in play, if(i) the player, his partner or either of their caddies:
• lifts or moves the ball,
• touches it purposely (except with a club in the act of addressing the ball), or
• causes the ball to move, or
(ii) the 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 covered by sand, in the replacement of Loose Impediments moved in a Hazard while finding or identifying a ball, in probing for a ball lying in water in a Water Hazard or in searching for a ball in an Obstruction or an Abnormal Ground Condition – 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.

Exception: If it is known or virtually certain that the player did not cause his ball to move, Rule 18-2b does not apply.

18-3. By Opponent, Caddie Or Equipment In Match Play
a. During Search
If, during search for a player’s ball, an opponent, his caddie or his equipment, moves the ball, touches it or causes it to move, there is no penalty. If the ball is moved, it must be replaced.

b. Other Than During Search
If, other than during search for a player’s ball, an opponent, his caddie or his equipment, moves the ball, touches it purposely or causes it to move, except as otherwise provided in the Rules, the opponent incurs a penalty of one stroke. If the ball is moved, it must be replaced.

(Playing a wrong ball – see Rule 15-3)

(Ball moved in measuring – see Rule 18-6)

18-4. By Fellow-Competitor, Caddie Or Equipment In Stroke Play
If a fellow-competitor, his caddie or his equipment, moves the player’s ball, touches it or causes it to move, there is no penalty. If the ball is moved, it must be replaced.

(Playing a wrong ball – see Rule 15-3)

18-5. By Another Ball
If a ball in play and at rest is moved by another ball in motion after a stroke, the moved ball must be replaced.

18-6. Ball Moved In Measuring
If a ball or ball-marker is moved in measuring while proceeding under or in determining the application of a Rule, the ball or ball-marker must be replaced. There is no penalty, provided the movement of the ball or ball-marker is directly attributable to the specific act of measuring. Otherwise, the provisions of Rule 18-2a, 18-3b or 18-4 apply.

*PENALTY FOR BREACH OF RULE:Match play – Loss of hole; Stroke play – Two strokes.

*If a player who is required to replace a ball fails to do so, or if he makes a stroke at a ball substituted under Rule 18 when such substitution is not permitted, he incurs the general penalty for breach of Rule 18, but there is no additional penalty under this Rule.

Note 1: If a ball to be replaced under this Rule is not immediately recoverable, another ball may be substituted.

Note 2: If the original lie of a ball to be placed or replaced has been altered, see Rule 20-3b.

Note 3: If it is impossible to determine the spot on which a ball is to be placed or replaced, see Rule 20-3c.