Terminology
 Group: a set of connected, likecolored stones on the board.
 Group size: the number of stones that a group contains. The smallest possible group has a size of 2. The diagram on the right shows a singleton and two groups with sizes 3 and 12.
 To drop: to place a stone on any empty space.
Rules
The game starts on an empty board. There are two players, white and black.

Alternative rules
The alternative rules are as above, with the following difference: On a turn, a player must perform exactly one of the following actions:
 Place one stone on an empty cell of the board.
 Place two stones on empty cells of the board, in such a way that the size of the largest group on the board at the end of the turn is the same as it was at the outset of the turn.
If you start up a Catchup game, you will have the option to choose these alternative rules, that were suggested by Luis Bolaños Mures.
Note
Catchup is a territory game using a somewhat unusual definition of territory. As long as they differ in size, players' territories consists of their largest groups. If these are the same size, then players' territories consists of their largest groups plus their 'next in size' (which may be the same size as the previous one), unless these territories are equal in size too, in which case the groups next in size are considered and so on, till an unequal territorycount results. A very strong incentive to always keep an eye on the overall situation, and not focus on the largest group alone.
Example game (original rules) Using the Catchup applet External links Catchup © Nick Bentley Java applet © Ed van Zon 