Eight jokers live in an American Mahjong set, and they bend the whole game around themselves. They're also the source of nearly every beginner rules dispute. Here is the complete law of the joker.
Where jokers work
A joker may represent any tile inside a group of three or more identical tiles: pungs (3), kongs (4), and quints (5). One real tile plus two jokers is a legal pung; one real tile plus four jokers is a legal quint.
Where jokers never work
Never in a pair. Never as a single tile. Never anywhere in a hand from the singles-and-pairs family — those hands are joker-free by definition, which is why they pay more. And never passed in the Charleston or courtesy.
The pair rule is the one to tattoo on your memory: it means pairs are the scarcest resource in the game, which quietly drives correct Charleston strategy (never break a pair) and hand selection (a hand needing two pairs you don't have is a long road).
The joker exchange
If any exposed group on any rack — yours or an opponent's — contains a joker, and you hold the actual tile the joker represents, you may exchange them on your turn: hand over the real tile, take the joker. The exposure stays legal; you gain a wild card.
This is the most under-used move in beginner play. Sharp players keep redeemable tiles precisely for this, and it's worth scanning every exposure on the table at the start of each of your turns.
Discarding a joker
Legal, and almost always wrong — with one elegant exception. A discarded joker is dead: it cannot be called or reclaimed by anyone. Late in a hopeless hand, when every real tile in your rack might hand an opponent the win, the dead joker is the one perfectly safe discard in the game. Defensive players know it; now you do too.