黑白棋

Reversi: rules, tactics and practical play

Trap and flip opposing discs against three levels of computer AI. This guide reflects the controls and win conditions in the current browser build. It is reviewed by the PocketJoy game-testing team and maintained by Long-Term New Media Limited.

Rules and winning conditions

You play black and the computer plays white, with black moving first. The only legal move is one that traps a continuous line of your opponent's pieces between your new piece and another of your own along any of the eight directions; every trapped piece flips to your colour, and if several directions qualify at once, all of them flip together. A square that would flip nothing is not a legal move. If you have no legal move, your turn is skipped automatically with a notice and nothing is required of you. The game ends when the board fills or neither side can move, and whoever has more pieces wins. Three computer difficulties are available, with the hard level evaluating positional value rather than merely counting pieces. The piece-count panel shows both sides live, and wins and games played are recorded on your device.

Match strategy

The most counterintuitive fact about Reversi is that having more pieces in the middlegame is no advantage and is often a liability. Pieces can be flipped back at any moment, so what really matters is position and mobility, meaning how many legal moves you have. The four corners are the strongest squares on the board: once taken they can never be flipped, and they anchor entire edges, so taking corners is the first priority of the game. Conversely, the three squares adjacent to a corner — the two beside it along the edges, often called C-squares, and the one diagonally inside it, the X-square — are the most dangerous places to play, since occupying them usually hands the corner to your opponent, and the X-square in particular is close to a losing move. Through the opening and middlegame, deliberately flip few pieces, keep your legal-move count above your opponent's, and force them into positions with no good options — a strategy called mobility restriction that lies at the heart of modern Reversi theory. When your opponent has no good move left, they must damage their own structure and offer you a corner or an edge. Watch for stable discs as well: pieces that can never be flipped, formed by unbroken lines running out from a corner along an edge. The endgame is really a contest of stable discs.

Frequent errors

The classic beginner error is always choosing the move that flips the most pieces. In Reversi that is close to the worst possible policy: the more you flip, the more of your pieces are exposed on more lines for your opponent to flip back, and your own mobility collapses because the pieces you just turned block the squares you would otherwise play. The second mistake is stepping onto an X-square or C-square beside a corner for a few extra flips, handing over the corner, which is usually the turning point of the whole game and is irreversible — a corner lost is never recovered. The third is watching the piece count rather than the structure. A twenty-piece lead in the middlegame can be overturned in the last ten moves, because one corner changing hands may flip an entire edge with it. The fourth is failing to exploit forced moves: when your opponent has only one or two legal squares, deliberately leaving that position intact compels them to play what you want, and that control is worth far more than a handful of extra discs.

Who it's for

Suited to players who like strategic board games but do not want to spend long learning rules. Reversi takes two minutes to explain yet has considerable depth, and its less-is-more character keeps it interesting for a long time while reliably giving newcomers the wrong instincts to unlearn — a process that is itself enjoyable. The modern version was commercialised in 1971 by Goro Hasegawa in Japan under the name Othello, taken from Shakespeare's play because the constantly reversing discs echo the drama's shifting suspicions, though its predecessor Reversi existed in England as early as the 1880s. It holds a place in the history of artificial intelligence too: in 1997 the program Logistello defeated the reigning world champion Takeshi Murakami six games to nil, an early marker of computers surpassing humans at board games. A game runs about ten minutes.

Feature highlights

Three computer difficulties are available, with the hard level weighting positional value — corners, edges and X-squares — rather than merely counting pieces, so it does not make the beginner's greedy-flip error. The piece-count panel shows both sides live, though bear in mind that a middlegame lead is often a bad sign in Reversi, so the panel matters mostly for confirming the result at the end; understanding that keeps the numbers from misleading you. The status line reports whose turn it is and whether the computer is thinking, and when a side has no legal move the turn is skipped automatically with a notice, so the game never appears frozen and you never wonder whether you missed an input — particularly important here, since skipped turns are a common and entirely legal occurrence that newcomers easily mistake for a fault. A record panel accumulates wins and games, and the result screen shows the final count, such as 38 to 26, so you can see how comfortable a win or how narrow a loss was. A restart button is always available.

Controls and match records

Tap a legal square to play; illegal squares simply do not respond, so the rules enforce themselves and beginners need not work out where they may move — a substantial help in Reversi, where judging legality means checking eight directions, which is slow and easy to get wrong by hand. Mouse and touch behave identically, and on a phone the board scales to a comfortable one-handed size. Flipped pieces change appearance clearly, so you can see which discs a move turned and along how many directions, which helps considerably in understanding positional value. A restart button is always available. Your record is stored in the browser's local storage on your own device, with no account and nothing uploaded to any server; clearing browser data removes it. The language button switches the interface between Traditional Chinese and English instantly, updating every prompt and result string.

Rules Q&A

The advanced ideas are collected in Reversi Strategy: Corners, Danger Squares and Mobility.

Reviewed by the PocketJoy game-testing team · Operated by Long-Term New Media Limited · hklongtermad@gmail.com