Onyx (game)

Onyx gameboard and starting position

Onyx is an abstract strategy board game for two players invented by Larry Back in 1995. The game features a rule for capturing, making it unique among connection games.

The Onyx gameboard is a grid of interlocking squares and triangles, with pieces played on the points of intersection (as in Go). The board has four sides, each comprising twelve intersection points. White tries to connect the two vertical (white) sides with an unbroken chain of white pieces, while Black tries to connect the two horizontal (black) sides with an unbroken chain of black pieces. The first to do so wins the game.

Onyx was featured in several issues of Abstract Games magazine edited by Kerry Handscomb.

Game rules

• The initial setup has four black pieces and four white pieces pre-placed (see illustration).
• Black moves first by placing a black piece on any empty intersection point. White follows suit.^{[note 1]} Turns continue to alternate.
• A player may place a piece on the midpoint of a square only when all four corner points of that square are unoccupied.
• Once placed, pieces do not move; but captured pieces are removed from the game.

Capture rule

A special rule for capturing allows a player to capture two enemy pieces in a single turn, if the following conditions are met:

• the two enemy pieces occupy opposite corners of a square;
• a third corner of the square is already occupied by a piece of the capturing player;
• the midpoint of the square is unoccupied.

The capture is executed by placing a piece on the remaining unoccupied corner of the square.

If the capturing move also simultaneously completes a second square on the board where the same conditions prevail, then the move results in the capture of four enemy pieces instead of two. (The possibility arises because each corner of a square on the Onyx board, with exception of corners at the board's edge, is also a corner of a second, adjoining square.)

Observations: non-repeating positions

Onyx has the interesting property that, despite having a capture rule, it seems positions never repeat in a normally played game. That is, if at least one player is trying to win then it does not appear to be possible to have an Onyx position where, after a number of moves have been made, some of which are captures resulting in the removal of pieces from the board, the game returns to the same position. It seems the only way a position can be repeated in Onyx is if both players conspire to bring this about. However, while experience indicates that positions do not repeat, it's not obvious why this is so. In fact, it may be possible to construct an Onyx position where, with correct play, the position will repeat after a number of moves. But such a position has never been discovered and it may be the case that it's impossible to create one. Therefore, one interesting challenge with Onyx is to construct a position that repeats with correct play or to prove that it is impossible to do so.

Notation

Each intersection point on the Onyx board, except for midpoints of squares, is described by a letter followed by a number in a zig-zagging coordinate system. The midpoint of a square is described by two letters followed by two numbers that uniquely identify the square's corners. "*" following a notated move indicates that one pair of pieces was captured; "**" following a move indicates two pairs were captured.

Variations

• Instead of the initial setup shown, players can begin the game with the board empty.^{[note 2]}
• The Onyx gameboard can be varied in size, with more or less than 12 interesection points per side.^{[note 3]}

See also

• Havannah
• Hex
• TwixT

References

Bibliography

Further reading

External links

• Back, Larry (Winter 2000). "An Original Connection Game". Carpe Diem Publishing. pp. 9–12. ISSN 1492-0492. Retrieved 2 May 2013. 
• The Game of Onyx introduction to the game by Dariusz Stachowski
• Gamerz.net play Onyx on Richard's PBeM Server
• Onyx at BoardGameGeek