Welcome to CHAOS;JAN — The Cosmic Deck.

CHAOS;JAN — The Cosmic Deck is played alongside regular riichi mahjong play. Since it only adds, it can be played with almost any varient. The recommended base-ruleset is regular 4 player mahjong or 3 player without nukidora (since a card enables nukidora later).

Note: These rules are for the original version of CHAOS;JAN. New versions of C;J have a completely different set of rules.


  1. Set Up
  2. Drawing Cards
  3. Modifiers
  4. Events
  5. Terminology/Language

Set Up

Before playing, choose which deck to play with (as of February 2022, there is only one deck to chose from, namely: the Cosmic Deck, but a second deck, World;Breaker is planned). Shuffle the cards thoroughly before playing (on Mahjong Horoscope, this is done automatically).

If you are using cards (ie. Tabletop Simulator): After shuffling, designate 3 spots for the cards. 1) The Library 2) Discards 3) Active Modifier. Make sure these spots are easily visible by all players and do not disrupt the mahjong game. Place the shuffled deck face down in the Library spot.

If you are using Mahjong Horoscope: Open the Cosmic Deck tool on a computer or phone. Refresh the page or press "Reshuffle deck" on the first card draw. Press "draw card" for the rest of the draws.

Use counters or dice to keep track of the number of draws each player has made that hand.

Drawing Cards

On each player's turn, before they draw a tile, that player may pay to roll a die for a chance to draw a card.

The player draws a new card if the die lands on an odd number.

The amount the player pays for each die roll is 1000 x 2^n where n is the number of times that player has rolled the die before during this hand.
(The price starts at 1,000 then doubles for that player for each time they rolled.)

After the player has paid to roll, they should use something to indicate how many times they've rolled this hand.

At the end of each hand, all players should reset their counts back to zero.

If a player does not have enough points to pay for an additional dice roll, they may not roll the dice.


When the first modifier is revealed, it should be placed in the Active Modifier spot. This shows which modifier's effect is currently active.

If the modifer has an effect that starts with "When this is revealed...", such as with Beginning of the Cosmos, that portion of the effect should take place before the player draws their tile.

If another modifer is revealed, discard the current Active Modifier, then place the new modifer in the Active Modifier spot.

Only one modifier can be active at once, modifiers in the discard pile do not affect the game.

If a card has "Sanma: ... Suuma: ...", then the card has different effects for if the game is played with 3 (sanma) or 4 (suuma) players. Only preform the action listed for the number of players your game has.


Events trigger when they are first revealed. Any actions on the card are preformed before the player who rolled draws a tile.

Some events's effects may linger beyond the instant the card is revealed, such as with Go Fish. Be sure to follow the event card's effect until it expires.

Whenever a card says "you" (such as Riichi Robbery), it refers to the player who revealed the Event.

Failure to preform an event will result in a dead hand and chombo (for whichever player failed to do the event, not necessarily the player who revealed the card). The hand does not restart (dice counters are not reset).

If a card has "Sanma: ... Suuma: ...", then the card has different effects for if the game is played with 3 (sanma) or 4 (suuma) players. Only preform the action listed for the number of players your game has.


You — The player who rolled and revealed a card.

Deck — The remaining cards in the game (ie. not the active modifier or discarded cards).

Hand — When refering to time (ie. "For this hand..."), the current round being played. A hand ends when the game goes to ryuukyoku (exhaustive or abortive draw), a player wins, or a chombo occurs that resets the hand.

Points on the table — Tenbou/Point sticks on the table refers to points such as riichi bets that are claimed when a player wins a hand.

Aotenjou — A scoring system where scores are not limited with Mangan. Instead, all hands are scored using han/fu and the normal formula (fu x 2^(2+han)). Yakuman yakus are scored as 13 han for each Yakuman. Kokushi Musou is scored as having 40 fu.

Revealed Tiles — Revealed tiles are tiles that are still in the hand, but opened for the table to see. Usually, these tiles stay revealed unless something unreveals them.

Yakitori — Yakitori is a game state a player is in by default. They lose yakitori status once they win a hand by ron or tsumo. Some mahjong sets come with yakitori markers which can be flipped over to show that a player is no longer yakitori.