Rule 24: Obstructions.

24-2 Immovable obstruction.

(a) Interference.
Interference by an immovable obstruction occurs when a ball lies in or on the obstruction, or when the obstruction interferes with the player's stance or the area of his intended swing. If the player's ball lies on the putting green, interference also occurs if an immovable obstruction on the putting green intervenes on his line of putt. Otherwise, intervention on the line of play is not, of itself, interference under this rule.
(b) Relief.
Except when the ball is in a water hazard or a lateral water hazard, a player may take relief from interference by an immovable obstruction as follows:
(i) Through the green: If the ball lies through the green, the player must lift the ball and drop it without penalty within one club-length of and not nearer the hole than the nearest point of relief. The nearest point of relief must not be in a hazard or on a putting green. When the ball is dropped within one club-length of the nearest point of relief, the ball must first strike a part of the course at a spot that avoids interference by the immovable obstruction and is not in a hazard and not on a putting green.
(ii) In a bunker: If the ball is in a bunker, the player must lift the ball and drop it in accordance with clause (i) above, except that the nearest point of relief must be in the bunker and the ball must be dropped in the bunker.
(iii) On the putting green: If the ball lies on the putting green, the player must lift the ball and place it without penalty at the nearest point of relief that is not in a hazard. The nearest point of relief may be off the putting green.