In addition to the standard rules to Japanese mahjong, additional "house rules" and variations may be applied to the game. Application of these rules are at the discretion of all involved. Rules of this nature are agreed upon by the players, and as a subsequence, the players go by these modified rules accordingly.
Variation on standard rules
Even though the rules are standardized, even some rules are subject to some variability based on interpretation, formality, time, and preference.
Formal settings and the online environment randomly draw the wind tiles. Prior to the start of a game, one of each type of wind tile is set aside for random draw; and players are seated according to the drawn winds. This typical draw process simply features 4 face-down wind tiles, but an older, more complicated variant with multiple dice rolls exists.
For casual play, the seating order may be disregarded, and players simply take a chair. One player may take the initiate to roll the dice for the first dealer position, and the game begins.
For casual and formal games, the player initially assigned East rolls the dice to determine the first dealer, and the game proceeds from there.
For computerized games, this process is not seen and works in the background. With today's processing speed, a computerized game begins as the tiles are initially drawn.
Variability involves when to flip the kan dora when a player invokes a kan. In casual play and in other various implementations, the timing of the flip of the kan dora is irrelevant; and generally, the tile is flipped immediately after a call. Especially in Tenhou.net, a slight difference in this rule exists, where the kan dora is flipped immediately after a closed kan. In the case of open kans, the tile is flipped after the discard.
Red dora are completely optional. They can be completely omitted, and they're typically "red 5's". However, other tiles can be marked as red, including the white Haku tile.
Atamahane, or head bump, is a specialized rule, that prevents multiple ron. With atamahane, either the first winning player according to the seat rotation, or the last player, will take precedence over the other(s). When atamahane is not used, both players will score points from the discarder.
Three rons on the same tile may be handled via atamahane, be scored as a triple ron, or cause an abortive draw.
Even in some Japanese professional mahjong circles, the usage of negative points is allowed. Even with point sticks, negative points can be tracked manually with pen and paper. Usually, the event of a player falling into the negatives results in an automatic end to the game.
The procedure for dice rolling may be modified. This applies to both determining the dealer and/or determining the wall break. Sometimes, the dealer is determined by multiple dice rolls. Initially, one player rolls to determine the player, who rolls for the dealer. Naturally, then that player's dice roll determines the first dealer position; and the game proceeds from there. As for wall breakage, two rolls may be used, where the dealer determines, which player wall is broken. Then the "owner" of the wall rolls the dice to determine the actual break position.
Some aspects of the yaku themselves may be subject to some modification.
In addition to the standard list of yaku, some extra set of yaku may be included for added variety.
Tanyao is a very simple and easy yaku to score, especially when it done with an open hand. To increase and slow down the speed of tanyao development, some rules may restrict this yaku to closed only.
Renhou is a standard yaku. However, it's value can count either as a yakuman or as a mangan, or not exist at all.
Some rules may impose a limit on multiple yakuman by disallowing them altogether. In addition, some yakuman are considered to be double, given specific conditions. For example, a 13-sided wait for kokushi musou is a double yakuman, but this may be relegated to just a single yakuman barring the rules applied. In addition, even if a player develops a hand composed of different yakuman, the point value may be capped to that of a single yakuman.
House rules are specialized rules implemented by the host.
Two yaku/han minimum
Instead of just a one-yaku minimum, the minimum may be increased to two for higher scoring play. This minimum can be reached with one yaku worth multiple han (such as chinitsu), or 2 yaku worth 1 han each (like tanyao pinfu). This disables the tactic of cheap and fast hands in favor of hands with greater value, and also prevents open hands with a large score solely due to dora. Some rules apply it only when there are 5 honba on the table.
Yakitori is a points penalty assigned to any player, who fails to win a hand during the game. This encourages all players to seek at least one win of a hand, rather than simply employing the defensive game throughout. The points penalty may vary.
Added penalties and bonuses
Additional penalties and bonuses may be added to increase point exchanges. For example, if a player scores a yakuman, then a player may receive a yakuman bonus.
Sometimes, players may reach tenpai without yaku. However, a winning tile may result in giving a player the yaku required to win. Most house rules may allow the win, while some may not.