Rule 15. Wrong Ball; Substituted Ball
A "wrong ball" is any ball other than the player's:
a. Ball in play,
b. Provisional ball or
c. Second ball played under Rule 3-3 or Rule 20-7b in stroke play.
Note: Ball in play includes a ball substituted for the ball in play,
whether or not such substitution is permitted.
A player must hole out with the ball played from the teeing ground
unless a Rule permits him to substitute another ball. If a player
substitutes another ball when not so permitted, that ball is not a
wrong ball; it becomes the ball in play and, if the error is not corrected
as provided in Rule 20-6, the player shall
incur a penalty of loss of hole in match play or two strokes in stroke
play. (Playing from wrong place-see Rule 20-7.)
15-2. Match Play
If a player plays a stroke with a wrong ball except in a hazard,
he shall lose the hole. If a player
plays any strokes in a hazard with a wrong ball, there is no penalty.
Strokes played in a hazard with a wrong ball do not count in the player's
score. If the wrong ball belongs to another player, its owner shall
place a ball on the spot from which the wrong ball was first played.
If the player and opponent exchange balls during the play of a hole,
the first to play the wrong ball other than from a hazard shall lose
the hole; when this cannot be determined, the hole shall be played
out with the balls exchanged.
15-3. Stroke Play
If a competitor plays a stroke or strokes with a wrong ball, he
shall incur a penalty of two strokes, unless the only stroke
or strokes played with such ball were played when it was in a hazard,
in which case no penalty is incurred. The competitor must correct
his mistake by playing the correct ball. If he fails to correct his
mistake before he plays a stroke from the next teeing ground or, in
the case of the last hole of the round, fails to declare his intention
to correct his mistake before leaving the putting green, he
shall be disqualified. Strokes played by a competitor with
a wrong ball do not count in his score. If the wrong ball belongs
to another competitor, its owner shall place a ball on the spot from
which the wrong ball was first played.