Ancient Chinese game of Black vs White
Each of the two sides corresponds to one player; they are referred to here as light and dark after the sides of Othello pieces.
(image 1 - see Screenshots)
Originally, Reversi did not have a defined starting position. Later it adopted Othello's rules, which state that the game begins with four markers placed in a square in the middle of the grid, two facing light-up, two pieces with the dark side up. The dark player makes the first move.
Dark must place a piece with the dark side up on the board, in such a position that there exists at least one straight (horizontal, vertical, or diagonal) occupied line between the new piece and another dark piece, with one or more contiguous light pieces between them. In the below situation, dark has the following options indicated by transparent pieces:
After placing the piece, dark turns over (flips, captures) all light pieces lying on a straight line between the new piece and any anchoring dark pieces. All reversed pieces now show the dark side, and dark can use them in later movesâunless light has reversed them back in the meantime. If dark decided to put a piece in the topmost location (all choices are strategically equivalent at this time), one piece gets turned over, so that the board appears thus:
(image 4) Now light plays. This player operates under the same rules, with the roles reversed: light lays down a light piece, causing a dark piece to flip. Possibilities at this time appear thus (indicated by transparent pieces):
Light takes the bottom left option and reverses one piece:
Players take alternate turns. If one player cannot make a valid move, play passes back to the other player. When neither player can move, the game ends. This occurs when the grid has filled up, or when one player has no more pieces on the board, or when neither player can legally place a piece in any of the remaining squares. The player with the most pieces on the board at the end of the game wins.
And that's all you need to know to play Reversi / Othello
Strategy A beginner often looks for the move that will reverse the greatest possible number of pieces, trying for immediate numerical advantage. For unsophisticated players, this strategy works quite well and will win a majority of games as long as the player thinks at least a few turns in advance. As the experience of the opponent increases, this strategy becomes ineffective. Instead of numerical advantage, the key elements of successful Reversi strategy are corners, mobility, edge play, parity, endgame play and looking ahead.
Corners Corner positions, once played, remain immune to flipping for the rest of the game (because there is no other opposite color behind them to create a flip); thus a player could use a piece in a corner of the board to anchor groups of pieces (starting with the adjacent edges) permanently. Therefore, capturing a corner often proves an effective strategy when the opportunity arises. More generally, a piece is stable when, along all four axes (horizontal, vertical, and each diagonal), it is on a boundary, in a filled row, or next to a stable piece of the same color. Grabbing a corner prematurely may be a mistake, however, if in doing so the player leaves "holes" along the edge. These holes can be filled by the opposing player and could result in capture of some or all of the pieces along that edge. This renders occupying the corner largely useless.
Mobility An opponent playing with reasonable strategy will not so easily relinquish the corner or any other good moves. So to achieve these good moves, you must force your opponent to play moves which relinquish those good moves. The best way to achieve this involves reducing the number of moves available to your opponent. If you consistently restrict the number of legal moves your opponent can make, then sooner or later they will have to make an undesirable move. An ideal position involves having all your pieces in the center surrounded by your opponent's pieces. In such situations you can dictate what moves your opponent can make.
When moves seem equal with respect to what moves you will leave yourself and your opponent, playing a minimum piece strategy will tend to give you an advantage, because minimizing your discs will tend to leave fewer discs for your opponent to flip in subsequent moves of the game. One should not play the minimum disc strategy to an extreme, however, as this also can quickly lead to a lack of mobility.
Edges While playing pieces to edges of the board may seem sound (because they cannot be flipped easily), this strategy can often prove detrimental. Edge pieces can anchor flips that influence moves to all regions of the board. This can poison later moves by causing players to flip too many pieces and open up many moves for the opponent. However, playing on edges where an opponent cannot easily respond drastically reduces possible moves for that opponent.
The square immediately diagonally adjacent to the corner (called the X-square), when played in the early or middle game, typically guarantees the loss of that corner. Nevertheless, such a corner sacrifice is sometimes played for some strategic purpose (like retaining mobility). Playing to the edge squares adjacent to the corner (called the C-squares) can also be dangerous if it gives the opponent powerful forcing moves.
In general, edge play in the early and middle game is to be avoided, unless players can gain larger concessions in terms of mobility or a mass of unflippable pieces.
A good rule of thumb is to keep pieces grouped together in the middle of the board and minimize tangents formed by a player's own pieces. This strategy leads to the greatest mobility.
Parity Parity is one of the most important parts of the strategy. In short, the concept of parity is about getting the last move in every empty region in the end-game, and thereby increasing the number of stable discs.
The concept of parity led to a change in the perception of the game, as it led to distinct strategies for playing black and white. It forced black to play more aggressive moves and gave white the opportunity to stay calm and focus on keeping the parity. As a result the opening books and mid-game were focused on black being the "attacker" and white being the "defender".
Another side effect of parity is that black should try to complicate the game whereas white should seek to simplify it. It is easier to maintain parity in a simple position.
The concept of parity also controls how edge positions are played and how edges interact.
Endgame For the endgame (the last 20 or so moves of the game) the strategies will typically change. Special techniques such as sweeping, gaining access, and the details of move-order can have a large impact on the outcome of the game. At these late stages of the game no hard-set rules exist. The experienced player will try to look ahead and get a feel for what will lead to the best final outcome.
- Ian S