Worldwide, many different game variants exist. Currently, Kajongg supports two variants of Classical Chinese, one as played in Germany and one as played in Great Britain.
This description does not go into details and as such matches both variants.
However most of the basic rules are the same in most variants.
Since the program Kajongg knows how to play, this description only contains things that might not be obvious.
If you want to learn Mah Jong in more detail, please consult the bibliography.
Each player tries to collect tiles such that she has a winning hand. On achieving this, she calls "Mah Jong". In a winning hand, the tiles are normally grouped into five melds: Four melds with three or four tiles each and a fifth meld consisting of two equal tiles. A normal meld has three or four identical tiles (called Pung and Kong respectively) or three successive tiles like Bamboo 3, Bamboo 4, Bamboo 5 (called Chow).
Initially, the player with the East wind (as indicated on the wall) has one more tile than the other players. East begins by discarding an unwanted tile.
Whenever a tile has been discarded, other players can claim it by saying what they can complete with that tile (Chow, Pung, Kong, Mah Jong). Only the next player can claim a Chow - the other claims are possible for all players. A meld completed by a claim will be exposed - made visible to all players.
If nobody claims the discarded tile, the next player, South (and then West and North) gets a tile from the wall and discards an unwanted tile.
There are several rules about when a Pung can be extended to a Kong (by claiming the forth tile or by getting it from the wall) - these rules are not explained here. But Kajongg of course knows them and makes sure they are respected. If a Kong is declared, the player automatically gets a replacement tile from the wall - otherwise it would not be possible to build the five melds as mentioned above.
When a player claims Mah Jong or we end up in a draw, a hand is completed. Now, except for draws, the winds rotate among the players. This is repeated until every player once had every wind. This entire unit is called a round. An entire game, also called year, consists of four such rounds: East wind round, South wind round, West wind round, North wind round. The wind of a round is also called prevailing wind, it can result in higher scores.
So a full game (a year) consists of at least 16 hands. Since the winds do not rotate for draws or when the East player says Mah Jong, a higher number of hands is normally needed for a full game.
A game can contain eight bonus tiles (unless the rules exclude them). They are exposed separately on the right side of the player region as they are never used for building melds. They do however generate bonus points.
If a player gets a bonus tile, she will automatically also get a replacement tile from the wall.
At the end of a hand, the score for each player is computed. First, basis points are counted like 2 points for a simple exposed pung or 20 points for winning. Next, another set of rules is applied - those rules can double the points like one double for a meld of dragons. When the score for each hand is known, differences are paid between players - while the winner gets its full score paid from all other players. East always gets and pays twice her score.
Of course Kajongg knows about all applicable rules but a good player knows what combinations get what score so she can optimize. While learning it is recommended to activate the window .
The exact scoring rules often vary - in Kajongg you are free to change them with the the ruleset editor.
The ruleset editor also shows you all available rules and their meaning. Place the mouse over them and for the more exotic rules help text will appear.