Onyx (game)

Lore & Background

Onyx was invented by Larry Back in 1995. The game is played on a grid of interlocking squares and triangles, with pieces placed on intersection points. Black aims to connect the two horizontal sides with an unbroken chain of black pieces, while White aims to connect the two vertical sides with an unbroken chain of white pieces. The first player to do so wins. The initial setup has four black and four white pieces pre-placed. Players alternate placing stones, with Black moving first. The pie rule can be invoked, giving White a one-time option to switch sides after Black's first move.

Reader's Guide

Onyx is notable among connection games for its capture rule, which allows a player to capture up to two enemy pieces in a single turn by completing a square under specific conditions. A double-capture can capture four enemy pieces if the move simultaneously completes two squares. The game has the interesting property that positions never repeat in normal play, though it is not proven whether a repeating position is impossible. Onyx was featured in Abstract Games magazine and has variations including an open variation with an empty board and larger board sizes such as 16×16. The game is played on Richard's PBeM Server and at the Gorrion gaming site.

Did You Know?

• Onyx was invented by Larry Back in 1995.
• The capture rule allows capturing up to four enemy pieces in a single move.

Board Geometry & Core Objective

Onyx was created by Larry Back in 1995 as a two-player abstract strategy game. Its playing surface is distinctive: a grid formed by interlocking squares and triangles, where pieces are placed at the intersection points rather than within cells—a layout reminiscent of Go. Each edge of the board contains twelve such points. The two players pursue opposing connection goals: Black aims to form an unbroken chain linking the two horizontal edges, while White works toward connecting the two vertical edges. Whoever completes their chain first claims victory. The game begins with four black and four white pieces already positioned on the board, and Black makes the opening move. To balance the first-mover advantage, White may invoke the pie rule, opting to swap sides after Black's initial placement. A subtle placement restriction governs the midpoints of squares: a stone can only be set at a square's center if all four of that square's corners remain empty. Once a piece is placed, it stays put unless it becomes the target of a capture.

The Capture Mechanism

What sets Onyx apart from other connection games is its capture mechanism, which introduces a tactical layer rarely seen in this genre. A player may remove up to two opposing pieces in a single turn, but only when specific geometric conditions align: the two enemy pieces must sit on opposite corners of a square, a third corner of that same square must already hold a piece belonging to the capturing player, and the square's midpoint must be vacant. The capture is executed by placing one's own piece on the fourth, still-empty corner. The geometry of the board enables an even more dramatic possibility: because most interior corners belong to two adjoining squares simultaneously, a single placement can satisfy the capture conditions for two squares at once, removing four enemy pieces in one move. This double-capture, however, is fragile. If the opponent captures first in one of those squares, the corner that anchored the second capture is removed, and the double-capture becomes impossible. This interplay of timing and geometry gives Onyx a depth that purely connection-based games typically lack.

The Non-Repeating Position Mystery

One of Onyx's most intriguing mathematical properties is that, under normal competitive play, board positions appear to never repeat. Despite the game featuring a capture rule that removes pieces from the board, it seems virtually impossible for the arrangement of stones to cycle back to a previously seen configuration when at least one player is actively trying to win. The only scenario in which a repetition might occur is if both players deliberately conspire to recreate an earlier setup. Yet no one has been able to explain definitively why this non-repetition holds. It remains an open question whether a position could be constructed in which correct play inevitably leads back to the same arrangement after a sequence of moves and captures. No such position has ever been discovered, and it may well be that creating one is impossible. This unresolved challenge—either find a repeating position under optimal play or prove that no such position can exist—stands as one of the most fascinating open problems in Onyx's small but devoted community.

Publication, Variations & Community Life

Since its invention, Onyx has found a home in the world of abstract gaming through the pages of Abstract Games magazine, edited by Kerry Handscomb. Larry Back published the original rules in the Winter 2000 issue, followed by a strategy-and-tactics deep dive in Summer 2001 and a formal game analysis in Autumn 2002, all under Carpe Diem Publishing. The game has also been featured in additional issues of the magazine, giving it sustained visibility among abstract-game enthusiasts. Players can modify the experience through several variations: the Open Variation drops the four pre-placed pieces of each color and starts from an empty board, while the board size itself can be expanded to sixteen points per side, the next size that still accommodates the central starting pieces. Online, Onyx is playable on turn-based platforms such as Gorrion and Richard's PBeM server on Gamerz.net, supporting both standard and open rules on 12×12 or 16×16 boards. Back's related game Diamond, which uses a similar tessellated board, further situates Onyx within a small family of geometric connection games.

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