An American set has 8 jokers. They're wild, but with limits, and those limits decide a lot of hands.
Where jokers work
A joker can stand in for any tile in a group of three or more of the same tile: a pung (3), kong (4), quint (5) or larger. A pung of three jokers is legal too.
Where jokers never work
- Pairs and singles. If the card calls for a pair of 5 Dots, it has to be two real 5 Dots.
- Singles & Pairs hands. That whole section of the card is built from singles and pairs, so no jokers and no exposures at all.
- Singles inside a hand, like the N, E, W and S in a NEWS group or the digits in a year. Each is a single tile, so no jokers there.
- The Charleston. Jokers can't be passed.
Swapping for a joker
Experienced players do this constantly. On your turn, look at every exposure on the table. If one has a joker standing in for a tile you're holding, you can trade your real tile for the joker. You can do it with your own exposures too. Then use that joker wherever you need it.
Watch the timing: swap on your turn, after you draw (or right after you call and expose a discard), and before you discard. Reaching for a joker before you've drawn can get your hand called dead.
Jokerless bonus
Win without any jokers and every payment doubles. The exceptions: Singles & Pairs hands can't use jokers in the first place, so their bonus is already built into the value printed on the card, and the bonus doesn't apply to Quints, which need jokers. Run any combination in the payout calculator.
Questions
Can you use a joker for a pair in mahjong?
No. Jokers only count inside a group of three or more identical tiles (a pung, kong, quint or larger). Pairs and singles must be the real tiles.
Can you pass jokers in the Charleston?
No. Jokers can't be passed during the Charleston.
When can you swap for a joker?
On your own turn. If any exposure on the table (including your own) has a joker standing in for a tile you're holding, you can swap your real tile for the joker.