Rules Explained: Your ball is unplayable - rule 28
You can declare a ball unplayable anywhere except in a water hazard, and the player is the sole judge as to whether her ball is unplayable. Declaring a ball unplayable results in a one-stroke penalty and there are three options which are easy to remember as they are A B and C.
A is for as nearly as possible – play a ball as nearly as possible from the spot where the original ball was last played.
B is for behind – drop a ball behind the point where the ball lay, keeping that point directly between the hole and the spot where the ball is dropped.
C is for club lengths – drop a ball within two club lengths of the spot where the ball lay but not nearer the hole.
A ball that is dropped can roll up to two club lengths from the spot on which it was dropped, even if this takes you off the line on which the ball was dropped or outside the two club-length area.
And you can clean your ball when using the unplayable rule or substitute another ball.