By Lew Blakey
Unfortunately, without the intent to do so, a player will sometimes make a stroke at a ball other than the one he struck from the tee. The question arises as to whether he has made a stroke at a wrong ball.
Consider the following situation.
In a stroke play competition, two players, A and B, strike their tee shots into trees bordering the fairway. When they arrive where the balls are likely to be, A sees a ball on a cart path, which he incorrectly identifies as that of B. A finds another ball adjacent to the cart path in heavy rough, says it is his and chips it onto the fairway. B lifts the ball on the cart path, takes relief according to obstruction Rule procedures and plays the ball into the fairway. When the players arrive in the fairway to play the balls again, they realize that they have exchanged balls, that is, the ball on the cart path was that of A and the ball in the rough was that of B.
They incorrectly conclude that each has played a wrong ball with a two stroke penalty but call for an official for confirmation. The official correctly makes the following ruling.
The ball played by A was B's ball in play and thus A has made a stroke at a wrong ball. He incurred a penalty of two strokes and must correct his mistake by playing the correct ball, which would be the ball played by B after B took relief from the cart path. If this ball is immediately recoverable, A must estimate the spot where it lay when it was lifted from the cart path by B and drop it as near as possible to that spot. If the ball is not immediately recoverable, then A may substitute another ball.
The ball played by B was a substituted ball because he dropped the ball with the intent of putting it into play. Since he did not know the location of his original ball at the time the substituted ball was dropped, he was required to proceed under the lost ball Rule, in which case the substitution was permitted. Since the substituted ball was not dropped at the spot required by the lost ball Rule, he played from a wrong place. In stroke play, he incurred a penalty of one stroke under the lost ball Rule and an additional penalty of two strokes for playing from a wrong place. Since his breach of playing from a wrong place was serious, he is subject to disqualification unless he corrects his error by playing again from the tee. The stroke from the wrong place doesn't count in his score for the hole.
This ruling clearly illustrates the difference between a wrong ball and a substituted ball. The key facts are  A made a stroke at a ball that he never put into play, thus it was a wrong ball, and  B made a stroke at a ball that was not his original ball but one that he put into play by dropping it, thus it was a substituted ball.