The Onyx board is a grid of interlocking squares and triangles, with pieces played on the points of intersection (as in Go). Each side of the board comprises twelve points. Black tries to connect the two horizontal (black) sides with an unbroken chain of black pieces, while White tries to connect the two vertical (white) sides with an unbroken chain of white pieces. The first to do so wins the game.
Onyx was featured in several issues of Abstract Games magazine edited by Kerry Handscomb.
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 point of the board. White follows suit.[note 1] Turns continue to alternate. A piece can be place on the midpoint of a square only if all four corners of that square are currently unoccupied. Once placed, pieces do not move. Captured pieces are immediately removed from the game.
The rule for capturing allows a player to capture two enemy pieces in a single turn. All of the following conditions must be met:
- the two enemy pieces occupy opposite corners of a square;
- a third corner of the square is already occupied by a piece owned by the capturing player;
- the square's midpoint 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.[note 2]
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.
Each point on the Onyx board, except for midpoints of squares, is notated 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.
An asterisk (*) following a notated move indicates that one pair of pieces was captured; two asterisks (**) indicates that two pairs were captured.
- Instead of the initial setup shown, players can begin the game with the Onyx board empty.[note 3]
- The Onyx board can be varied in size, with more or less than twelve points per side.[note 4]
- Or, since moving first conveys an advantage, the pie rule can be invoked. This gives White the one-time option to switch sides after Black's first move.
- This possibility arises since 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.
- However, the standard setup with pre-placed pieces "tends to sharpen the play and results in more interesting positions". (Back 2000:11).
- "A larger board results in more strategic depth but also a longer game. It seems a board [...] with twelve points along each side is ideal for this game." (Back 2000:11). Some puzzles have been composed for Mini-Onyx boards having six points per side (Back 2000:12).
- Back, Larry (Winter 2000). Kerry Handscomb, ed. "Onyx: An Original Connection Game". Abstract Games. Carpe Diem Publishing (4): 9–12. ISSN 1492-0492.
- Back, Larry (Summer 2001). Kerry Handscomb, ed. "Onyx: Strategy and Tactics". Abstract Games. Carpe Diem Publishing (6): 22–27. ISSN 1492-0492.
- Back, Larry (Autumn 2002). Kerry Handscomb, ed. "Onyx: Analysis of a Game". Abstract Games. Carpe Diem Publishing (11): 26–28. ISSN 1492-0492.