American Mah Jongg rules

1 The set

152 tiles.

Dragons belong to suits. Green goes with bams, red with cracks, white with dots. A card line calling for dragons in a given suit means that suit's dragon specifically, not any dragon.

The white dragon is "soap" and reads as zero in year hands. A line spelling 2026 is satisfied by 2, soap, 2, 6.

Flowers are interchangeable. Unlike Chinese mahjong, American play treats all eight as the same tile.

2 Hands and the card

A card is a set of winning hands. A hand is exactly 14 tiles, made of groups of identical tiles:

Jokers substitute in groups of three or more, never in singles or pairs. This follows from group size alone; no hand overrides it.

Jokers are not required to be a last resort. A player may use a joker even when the natural tile is in hand. Some products wrongly force naturals first (the "finesse" bug below).

How hands are written

A hand rarely names a concrete suit. It says "any one suit" or "any three suits". So hands are stored with suit variables A, B and C, which always resolve to different real suits. A hand mentioning only A is a one-suit hand; one mentioning A and B needs two suits. Matching tries every assignment.

Variables must be used in order - a hand may use A, or A and B, or A, B and C, but never B alone or A and C. Two spellings of the same requirement would otherwise disagree during matching.

Consecutive-run hands say "any run of numbers", so a rank is either a literal 1-9 or the variable n with an offset. A hand using n, n+1, n+2 and n+3 admits n from 1 to 6, because n+3 must not exceed 9.

Dragons in hands

A line calling for dragons in suit A means that suit's dragon: green for bams, red for cracks, white for dots. A year hand's zero is always the white dragon (soap) regardless of the suit the rest of the year sits in, so it is stored as its own thing rather than as a dragon of some suit.

3 Matching tiles to a hand

A hand is not one fixed requirement. Its suit variables can map onto the real suits any way round, and its rank variable slides along its range, so one hand has many resolutions. Tiles are scored against every resolution and the best is kept - which is what a player does when they look at a line and think "could I do that in cracks instead?"

Distance is how many tiles still need to be acquired. Zero means complete.

Joker allocation

Within a resolution, jokers fill whatever natural tiles are missing from groups of three or more. Two rules govern this, and both are places real products go wrong:

1. A joker can never complete a single or a pair. A hand of singles and pairs cannot use jokers at all.
2. When two groups want the same tile and only one accepts jokers, natural tiles go to the group that cannot use one. Otherwise a joker gets spent where a natural tile would have served, and the hand looks further away than it is.

Where a hand could be completed either naturally or with a joker, the natural completion is reported, so a hand is never described as needing a joker it does not need.

4 Which hands are possible at all

A hand can be well formed and still impossible. The deck holds four copies of most faces, eight flowers and eight jokers, so a hand asking for more naturals than exist can only stand if jokers cover the shortfall.

The rule: a hand is impossible when the demand from groups that cannot take a joker exceeds the deck's supply of that tile. Ten flowers spread across five pairs is impossible; nine flowers across a kong and a quint is fine, because the ninth can be a joker.

Quints and sextets can never be made naturally. No tile exists five times, so a quint always costs at least one joker and a sextet at least two. A hand of two quints needs two jokers before play even begins.

Jokers are a shared pool, so a hand can be impossible even when every group looks fine on its own - two sextets and a pair need four jokers, and a hand needing more than eight cannot be completed at all.

5 The table

Four seats: east, south, west, north. The winds run counterclockwise around the table, so the player on your right is the next one in that list.

Play passes to the right. East is followed by south, then west, then north. This is easy to reverse by accident and invisible when wrong - a reversed table silently reverses the Charleston and produces games that look plausible but are a different game. It is pinned by test.

From any seat, the other three are exactly right, across and left, which are also the three directions of the first Charleston.

6 The deal

The deck is shuffled and dealt: fourteen tiles to the dealer, thirteen to everyone else. The dealer opens play by discarding. Ninety-nine tiles are left in the wall.

Deals are reproducible from a seed, so any game can be replayed exactly.

7 What a player can see

A player may reason from: their own concealed tiles, every exposure on every rack, and every discard. Nothing else.

Other players' concealed tiles and the contents of the wall are not visible, and no bot is permitted to consult them. Bots that appear to see through the wall are the single loudest complaint players make about existing products.

8 Concealed tiles and exposures

Tiles are either concealed in hand, seen only by their owner, or exposed face up on the rack after claiming a discard. Exposing tells the table roughly what you are building, which is why concealed hands are worth more.

An exposure keeps any jokers laid down in it, because a joker in an exposure can later be claimed by a player holding the natural tile it stands for.

9 The Charleston

Before the first discard, tiles are passed around the table. Nothing in any other mahjong variant resembles this.

Three tiles per pass, and all four players pass at the same time - you give tiles away without knowing what is coming back. A pass is therefore resolved as one atomic operation: every selection is checked before any tile moves, so one player's illegal pass cannot leave the table half-exchanged.

Jokers may never be passed. Not in the Charleston, not in the courtesy pass. A bot in a shipping product violates this.

The courtesy pass goes across only, and opposite players must exchange the same number of tiles - so east and west may swap two while south and north swap none.

Blind passes

A player who does not want to give up three of their own tiles may pass on tiles they have just received, face down, without looking. A tile held since the deal cannot be passed blind; that is simply an ordinary pass.

Blind tiles are recorded, because their sender never saw them - a replay reconstructing what a player knew has to leave them out.

Confirmed. A blind pass is permitted on the last pass of each Charleston - the left pass of the first, the right pass of the second - and a player may pass one, two or three tiles blind. Each pass still totals three tiles. Sources: Wikipedia, American mahjong and the Standardized National Mah Jongg Tournament Rules, rules 5E and 6. The implementation matched on first writing.

The tournament rules add a detail we do not model, since it concerns physical play: looking at a stolen tile costs the player ten points.

10 The turn loop

On your turn you draw a tile from the wall and then discard one.

Any player may claim a discard, not just the next one. This is the rule that most sharply separates American Mahjong from other variants: a discard is exposed to the whole table, and claiming it jumps the turn to the claimer, skipping everyone in between.

A discard may only be claimed to complete a set of three or more. You cannot claim a tile to finish a pair or a single unless it completes your whole hand. A discarded joker cannot be claimed at all.

Who wins a contested discard

1. A claim for mahjong beats a claim for an exposure.
2. Among equal claims, the seat nearest the discarder going right wins.

The winner lays the set face up, then discards, and play continues from their seat.

11 Joker exchange

On your turn you may hand over the natural tile a joker stands for and take that joker into your hand. This works on any exposure on the table, including your own.

Only exposures can be raided. A joker in a concealed hand is untouchable.

You must pick before you exchange. A turn begins by taking a tile - from the wall, or by claiming a discard and building the exposure. Exchanging for a joker before picking is one of the listed ways a hand is declared dead (tournament rules 7A, 14C, 19A).

A dead player's exposed jokers stay redeemable (rule 19B), as long as the exposure was legal when it was made. Being dead takes you out of the running, not your tiles off the table.

12 Dead hands

A hand is dead when no hand on the card can still be completed. Two things close hands off:

Exposures are irrevocable. Once a pung of five bams is face up, only hands containing a group that could be five bams remain. Every exposure must land on a distinct group - two exposed pungs of fives need a hand with two groups of fives, not one.

Tiles run out. A hand needing a pair of north winds is gone once all four are visible elsewhere. Groups of three or more survive this, because jokers can cover them; pairs and singles cannot.

The engine reports a reason per hand, not just a verdict, because the common case is that the app is right and the player wants to know why.

Confirmed. Three jokers together may form a group in their own right - "When 3 Joker tiles go together, they can be a composition" (Wikipedia). So an all-joker pung, kong, quint or sextet is legal, and the matcher is right to allow it. An exposure always holds at least one natural tile anyway, because the claimed discard is natural.

13 Settlement

Players settle with each other, not with a bank, and how the hand ended changes what everyone pays.

A hand won with no jokers pays double again (added 4 October 2026: "we should be using league rules"). If none of the winner's fourteen tiles, exposures included, is a joker, every payment above doubles: a jokerless win on a discard costs the thrower 4x and the others 2x, a jokerless self-drawn win costs everyone 4x. The bonus does not apply in Singles and Pairs, where no hand can hold a joker, or in Quints. Under NMJL social rules the winner must announce it; the engine applies it whenever it is earned. It is the house-rule option jokerless_multiplier (1 turns it off). Source: Sloperama, FAQ 19: "doubled again if the hand has no jokers", and "The jokerless bonus does not apply in Quints."

A dead hand still pays. Being out of the running does not excuse you from settling, which is exactly why a dead player should stop feeding the table.

This is the social and home-game settlement given on the NMJL card. Tournament scoring is a different system - cumulative points with bonuses for a self-picked win and for winning without jokers, penalties for throwing the winning tile and for a dead hand - and is not implemented here. See the Standardized National Mah Jongg Tournament Rules, rule 13, if a tournament mode is ever wanted.

If the wall runs out with no winner the hand is abandoned and no money moves.

14 Ending the hand

A hand ends when someone completes a line on the card - either by drawing the tile themselves, or by claiming a discard for mahjong - or when the wall is exhausted.

The heavenly hand. If the dealer's fourteen dealt tiles already complete a line, they declare immediately and the Charleston never runs.

A player holding a completed hand declares which line they have made. Where tiles satisfy more than one line, the higher-scoring one is declared.

A claim must leave you a tile to discard. After claiming you have to throw one, so a claim that would use up every concealed tile is illegal - your whole hand would be face up with nothing left to play. Found by a bot claiming its way into exactly that position.

15 House rules

Tables differ on 17 points. Every one is printed below with the rule as published and what it can be set to instead. Where you set them: on your own table, open Settings and the House rules section; a club sets them on its page for every table it runs; a group on its page.

Jokers

Charleston

Claiming

Settlement

Play