Quick answer
A joker can stand in for any tile in a group of three or more identical tiles, never in a pair, a single or a Singles and Pairs hand. Jokers can never be passed in the Charleston or courtesy pass. On your turn, after picking or calling, you may swap the matching natural tile for a joker in any exposure. A discarded joker is dead.
An American Mahjong set has eight jokers. This guide covers where they can and cannot go, when you may take one from an exposure, and what happens to one that is discarded.
Where jokers work
A joker may represent any tile inside a group of three or more identical tiles, such as a pung (3), a kong (4), or a quint (5). One real tile plus two jokers is a legal pung, and one real tile plus four jokers is a legal quint.
Where jokers never work
Never in a pair. Never as a single tile. Never in a group of different tiles, such as NEWS or the year, because a joker only stands in for a tile in a group of identical ones. Never anywhere in a hand from the Singles and Pairs section of the card, since every group there is a single or a pair. And never passed in the Charleston or the courtesy pass.
Every pair in a hand has to be built from natural tiles, and a discard can be called for a pair only as the tile that wins the hand. So any tile a pair still needs has to come from the wall or the Charleston, or be your winning tile, which is why a natural pair that fits a hand you can make is hard to replace.
Flowers, winds and dragons
Yes, inside a group of three or more. A joker can fill a pung, kong or quint of flowers, of one wind or of one dragon, the same way it fills a group of a numbered tile.
The group still has to be identical tiles. All eight flowers count as the same tile, so any flowers will do. Every tile in a dragon group is the same dragon, and every tile in a wind group is the same wind.
The usual limits hold. A joker never goes in a pair of flowers, winds or dragons, never stands in for a single one, and never goes in NEWS, which is four different winds.
Eight jokers, and why the count matters
An American set contains eight jokers. That number is worth holding in your head, because jokers are a shared pool rather than a personal allowance. Every one sitting in somebody else's exposure or lying in the discards is one that is not coming to you.
It also sets a ceiling on what the table can do. A card line asking for a quint of anything other than flowers needs at least one joker by construction, since a set has only four of each suited tile, wind and dragon, so when the jokers are gone the hands that depend on them go with them.
Counting them is a cheap habit. Two exposed across the table and one discarded means five unaccounted for, and a direction of yours that needs two is now competing for a thinner supply than it was at the deal.
What a jokerless hand is worth
Under League rules a hand won with no jokers in it is paid double, on top of any other doubling. A self-picked jokerless win is doubled twice, so all three opponents pay four times the hand's value. The exception is the Singles and Pairs section of the card, whose hands can never hold a joker and earn no double for going without.
Two details decide whether you collect. The hand is judged as it stands when you declare, so what matters is that no joker is in it at that moment. And the winner has to announce the hand as jokerless when declaring, or the double does not apply. Sanctioned tournament scoring uses points instead, adding twenty for declaring mah jongg with no jokers in the hand, with no such bonus on the Singles and Pairs group.
Throwing back a joker that would complete your hand, to wait for the natural tile, gives up a win you already have for a bigger one you may not get, and every turn you wait is another chance for someone else to go out first. The double costs nothing in one kind of position. The hand's only joker is in your rack, filling a group of three or more, and every tile you still need is for a pair or a single, which no joker can fill. If you then pick the natural tile for the joker's group, discarding the joker keeps every way the hand had to win, and a win from there pays double.
Jokers when a hand goes dead
A hand being declared dead does not freeze the jokers already sitting in its exposures. If the exposure was correct when it was made, the jokers in it can still be redeemed by anyone holding the natural tile, for the rest of the hand.
The exception is the one that caused the problem. If the exposure itself was what made the hand dead, it is not a valid exposure, and its jokers are not on the market.
If your hand is the one that died, your tiles are still doing work for your opponents. It is another reason the cost of a wrong exposure is larger than losing your own turn.
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 and take the joker. The exposure stays legal and you gain a wild card.
League rules set the order. Start your turn first, by picking from the wall or by claiming a discard and building its exposure, and only then exchange. Exchanging before you have picked or claimed makes your hand dead. You may not take an opponent's joker to finish the exposure you are building with a claimed discard. And the exchange has to come before your discard, because once a tile has been named, even partly, or has touched the table, it is discarded and can no longer buy a joker.
Each exchange costs you a real tile, so make it when the joker does more for your hand than the tile you hand over. It can also raise what you pay. A hand counts as jokerless if no joker is in it when mah jongg is declared, so taking the last joker from an opponent's exposures can leave their hand jokerless, and if they then win, every payment to them doubles. Checking the exposures for jokers you could buy is a habit worth building into every turn.
Discarding a joker
Legal. A discarded joker is dead, so nobody can call it or take it back. It also covers the discard before it, so that tile can no longer be claimed either.
That leaves three reasons to discard one. The hand you are making cannot use it, as in a Singles and Pairs hand. You want the hand jokerless for the double. That costs nothing once the joker's group is complete in natural tiles and every tile you still need is for a pair or a single. Anywhere else, the joker could still fill a tile the hand needs. Or the hand cannot be won, and a joker is the one discard nobody at the table can use.
Put it into practice
New to the game? Start the course at Lesson 1, Meet the Tiles. The first three lessons are free and need no account.
Already play? Have the coach read a real dealt hand for you, free. It names the direction the hand goes for, what to keep, what to pass, and the reasoning behind each call.
Knowing the joker rules and knowing when to spend one are different skills. Our AI coach reads a dealt hand and says which direction it points, free, no account needed.