Space mahjong
Space mahjong is a modified set of rules to mahjong originally conceived by the R18 game Itadaki jan-gari-anR. In space mahjong, terminals may "wrap around", so sequences of 912 and 891 become possible. In addition, honor tiles may be used in sequences. Another big difference in this ruleset is that you can call Chii from any player just like if you were calling Pon.
Tile groupings
Due to the wrap around effect, terminals and honors have much better tile efficiency than normal.
Numbered tiles
In regular mahjong, 1 and 9 cannot be part of the same sequence. In space mahjong, 9 may connect to 1 like any other number tile. Therefore, the terminals are no worse for tile efficiency than the middle tiles.
Honor tiles
With wrap-around effect, honor tiles become significantly important, for the fact that honor-related yakus are usually in high value, as well as they can now form mentsus efficiently.
Any three different tiles of kazehais / sangenpais can be formed as a valid shuntsu. That means all of the following sets are valid:
, 
, , , 
.
Any two different tiles of kazehais / sangenpais can be formed as a valid taatsu:
, ryanmen waiting for both 
and 
.- , kanchan waiting for both and .
, waiting for 
, can be interpreted as either kanchan or ryanmen.
Four different tiles of kazehais can be formed as a 4-sided tanki wait:
- , tanki waiting for .
Yaku
Tanyao
Tanyao is scored as normal. (Because terminals/honors are easier to use, though, it is less important than usual.)
Chiitoitsu
This yaku is not allowed in this ruleset.
Chanta / Junchan
Aside from honor-tile shuntsus, there are two more available combinations of numbered-tile shuntsus: 891 and 912. This makes chanta and junchan become much easier than in regular rules.
Agari:
Honroutou
Honors may be used in sequences, so it is possible to gain honroutou without toitoi or chiitoi.
The following hand shows how it is possible with Honroutou-nomi.
-
Agari:
Kokushi Musou
Opened kokushi is now possible, as honor tiles can be grouped as shuntsus.
-
Agari:
However, open kokushi is greatly reduced in value (e.g. 2 ~ 3 han) for balancing purposes.
Junsei Chuuren Poutou
Theoretically a 9-sided wait hand is no longer restricted to the regular form since numbered tiles can be wrapped around. That means such hand
-
Agari: (in regular rules)
can be interpreted as:
- Agari: (in space mahjong rules)
Though depending on rule details this may be recognized as a valid chuuren, or maybe not.