I'll presume that the reader is familiar with the rules and some basic concepts of Go. In 1975 Mark Berger invented 'Rosette', a Go variant played on the intersections (triple contacts) of a hexagonal board. His first idea was to simply apply the rules of Go and see how it worked out. As it turned out, regular concepts like 'ko' and 'seki' remained intact, but there was a big difference if a group was in 'atari', that is: if it had just one liberty left, like the white stone. In Go a point has four liberties and extending from a group in atari may increase the number of its liberties by 2. In Rosette an extension increases that number at most by 1, and this one is consequently taken to keep the group in atari.

The attacker has the choice of direction and may lead the head of the 'escaping' group towards the edge or even around towards its own tail, to die.
Of course things were balanced by the fact that both players suffer or enjoy this to the same extent, but Mark concluded rightly that it gave rise to too much tactical involvement to leave much room for any long term strategy. So he invented a safety mechanism up and above the implicit safety mechanism of having two 'eyes', and called it a rosette.
A rosette is formed by six stones of one color, occupying a small hexagon. A group containing a rosette lives unconditionally. This turned out to be a great improvement and I made it a key idea in shaping Medusa and Lotus.

Thanx Mark, wherever you are!

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Rules The game starts on an empty board. Play is on the hexagons. Only the lighter colored cells are part of the board, thus a cell in the center has four neighbours, one on the edge has three and a corner has two. Each player has enough bi-colored stones - black one side, white the other. Next to the board is a 19-point track with a marker in the center. It is there because refraining from putting a stone on the board at one's turn earns a point, indicated by moving the marker.

Placement and movement options

• White moves first, after which turns alternate. On his turn a player has two options:
  • He may place a stone on any vacant cell.
  • He may 'move' any or all of his groups.
• He may use either or both. If he uses both, placing a stone must precede movement.
  If he drops the first option he may move the marker one point towards his side.

Movement
For movement purposes a group is defined as two or more connected stones of the same color.

• Moving a group means taking one of its stones and moving it in a staight line over any number of friendly stones in an unbroken row, to land on the first vacant cell beyond.
• No group may move more than once in a turn.
• By movement groups may split or join.
• If a group moves to contact a friendly group that has not yet moved itself, the latter therewith loses its right to move in that turn. Thus the order of moving groups may make a difference.

Life and death

• A group lives unconditionally if it contains a 'rosette' - six stones around one of the board's 61 dark-colored cells.
• For capturing purposes a single stone is, by definition, also a group. A group without a rosette lives as long as it has at least one 'liberty', that is: one adjacent vacant cell. A group that is down to one liberty, and thus under immediate threat of being captured, is said to be 'in atari'.
• Assuming none of the groups involved is protected by a rosette, a group that loses its last liberty is captured and reversed immediately to show the opponent's color.
• Capture may result from placement or movement or both. If a capture is the result of placement only, the stones are reversed before the movement phase. Thus the group resulting from the capture has the right to move in the same turn. Groups captured by movement contain at least one stone that has moved, and may not move in the same turn.
• If placement results in one or more opponent's groups losing their last liberty, they are captured whether or not the stone placed has any liberties itself at the time of placement.
• If placement results in losing one's own last liberty without killing any opponent's group, the placement is suicidal and the player's own stone or group is reversed before the movement phase.
  Suicide is legal.

Object and counting

• The game ends by one player's resignation or if both pass completely on successive turns. In the latter case dead stones are reversed.
• The winner is now the player with the most territory.
  Territory consists of a player's number of stones on the board plus the number of cells totally surrounded by his stones
  The player whose side it is on also adds the number of points indicated by the marker.
• Seki may occur: empty points not totally surrounded by either player count for neither.

Examples

At the top black is safe. A white placement at A is suicidal and gives black a rosette, making movement at B (qs15) useless. If white plays at B, black must answer by completing a rosette at A himself on the next turn. At the bottom black threatens to connect by capturing the group on the bottom left side. It's white's turn however and the four black stones at the bottom are killed by a suicide placement at C, creating a five stone black group with two liberties, followed by two movements: ec1 and ec3.

This is a seki Neither player can occupy A or B without having his stones killed.

Strategy
In general sense Medusa strategy mirrors Go strategy, but there are differences.
Invasions in Medusa, including those unlikely to succeed, don't harm as long as the opponent must answer by placement. Failure only makes that the opponent gets some dead stones on cells that would have been part of his territory anyway, while the invader doesn't lose anything in trying. However, if the invading stone is so weak as to require no answer by placement, the invader loses a marker point.
The same holds for overdefensiveness: 'killing' already dead stones doesn't so much change the division of territory, but it gives the opponent the opportunity to refrain from placement and move the marker.

Medusa is highly efficient in that stones that lose their local importance in the course of a game, can often be moved to where they're needed more urgently.

Medusa and its support-act Lotus are featured in R. Wayne Schmittberger's 'New Rules for Classic Games' (John Wiley & Sons, Inc. New York; ISBN 0-471-53621-0).


How I invented ... Medusa
External links

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