Unplayable ball
You're the only one who decides whether your ball is unplayable. You can do this anywhere on the course except in a penalty area. Rule 19 then gives you three options, each with one penalty stroke. If your ball is in a bunker, there's an extra option with two penalty strokes to drop outside the bunker.
Golfregels App for iPhone and iPad
All definitions and rules at hand offline, with practice questions per topic.