The joker exchange is the rule that turns exposures into a market. Any joker showing on any rack can be bought with the real tile it stands for. Here is the mechanism, precisely, and the two or three ways players get it wrong.
What can be exchanged
Only a joker that is sitting in an exposed set on someone's rack, yours included, can be exchanged. Jokers in a player's hidden hand are untouchable. Jokers in the discard pile are dead and cannot be taken.
You exchange by giving the exact natural tile the joker is representing. If the exposure is a pung of six bams with a joker as the third six bam, only a six bam buys that joker.
When you can do it
On your own turn, after you have drawn (or called) and before you discard. You may make more than one exchange in a single turn if you hold more than one matching tile. You cannot exchange during someone else's turn, and you cannot exchange instead of drawing.
What happens to the joker
It goes into your hidden hand and is yours to use anywhere a joker is legal: in a pung, kong, or quint of any hand you are building. You then discard as normal. Your tile count does not change, since you gave one tile and received one.
Three ways it goes wrong
Offering a tile that does not exactly match the set: a six crak does not redeem a joker standing for a six bam. Trying to exchange for a joker in a pair: jokers cannot be in pairs, so there is never one there to take. And exchanging out of turn, which most groups treat as a dead-hand offence if tiles have been moved.
Why it matters strategically
Every exposed joker is a loan you can lose. Players holding the matching natural tile will take it, often late in the hand when it hurts most. So exposing a set that contains a joker is a bigger decision than exposing one made of naturals, and the later you can leave it, the safer it is.
Put it into practice
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Whether to expose a set with a joker in it depends on where your whole hand is going. Our AI coach reads your actual tiles and says which direction they point, free, with the reasoning.