Rule of the week

Where in the Official Rules of Golf is the three minute time limit on searching for a ball spelled out? In Rule 18-2, which covers balls lost or out of bounds. The rule includes this:

"A ball is lost if not found in three minutes after the player or his or her caddie begins to search for it."

Further, the Definitions section of the rule book includes this definition of "lost":

The status of a ball that is not found in three minutes after the player or his or her caddie (or the player’s partner or partner’s caddie) begins to search for it. If the search begins and is then temporarily interrupted for a good reason (such as when the player stops searching when play is suspended or needs to stand aside to wait for another player to play) or when the player has mistakenly identified a wrong ball: a) The time between the interruption and when the search resumes does not count, and b) The time allowed for search is three minutes in total, counting the search time both before the interruption and after the search resumes.