Trap discs on an 8 by 8 board and finish with the most. Play the computer on Easy, Medium or Hard, or take turns with a friend on one screen.
Reversi is a two-player game on an 8 by 8 board where you place a disc so that it traps a straight line of your opponent's discs between your new disc and another of your own, and every trapped disc turns to your colour. The board starts with two dark and two light discs in the centre, and dark moves first. A move is legal only if it turns over at least one disc. The line can run along a row, a column or a diagonal, so a single move can turn discs in up to eight directions at once. The discs between must all be the opponent's, with no gap and no edge in the way, and the line cannot wrap around the side of the board. On this page the squares you may play carry a teal dot, and the last disc placed has a teal ring. If you have no legal move you pass, and the status line says so; the other side then moves again. The game ends when neither side can move, which is usually a full board. A game can last at most 60 moves, because four of the 64 squares are filled at the start. The player with more discs wins, and equal counts are a draw. Squares are named with a letter for the column and a number for the row counted from the top, so the four opening moves for dark are d3, c4, f5 and e6.
Easy turns over the most discs it can, with extra weight on corners, and roughly one move in three it plays a random legal move instead, so it can be beaten by steady play. Medium searches three moves ahead and scores each position by square values, corners held, how many moves each side has, and how many discs touch an empty square. Hard uses the same scoring and searches up to six moves ahead, within a fixed limit on the positions it examines. When nine or fewer squares are empty, Hard stops guessing and plays the game out to the end, so it picks the move that leaves the biggest final lead, or the smallest loss. The search follows the shared rules of the page, so the computer never plays an illegal move; the tests check this over many random positions, and they check that Hard's endgame choice matches an exact search of the last nine squares. The same position and the same random numbers always give the same reply. The computer is not a perfect player: Hard can still lose to someone who controls the corners and keeps more moves available than it has.
Flipping many discs early is usually a mistake, because the discs you turn give your opponent more places to play. Count moves rather than discs: the side with more legal moves can choose a good one while the other side runs out. Corners matter most, because a corner disc can never be flipped. A flip needs your disc on both ends of the line, and a corner has no square beyond it. The squares next to an empty corner are risky, since playing there often lets your opponent take the corner. An edge disc can only be flipped along the edge, because no other line runs through it, so edges hold their colour better than the middle. Discs on the outside of a group touch empty squares and give your opponent moves, so try to keep your discs inside it. The number of possible move sequences shows how fast the game opens up: after 1, 2, 3, 4 and 5 moves there are 4, 12, 56, 244 and 1396. Near the end, each move fills one empty square, so count them: a move that leaves your opponent with no reply makes them pass, and you move again.
Click or tap a dotted square to play. With a keyboard, the arrow keys move between squares, Home and End jump to the ends of a row, and Enter or Space plays the square you are on. Squares are read out by name, for example d3, empty, legal move, and the computer's moves and passes are announced. Undo takes back your last move and, against the computer, its reply as well, so you are on the move again; it is off once the game has ended. The game in progress, your colour, the level and your win record against the computer are saved in this browser, so closing the tab does not lose them. The record counts wins, the current win streak and the biggest winning margin. Two players share one screen and take turns. This page has no online play, no clock, no hints and no sound, and flips are animated only if your device allows motion.
That player passes and the other side moves again. The status line names the player who passed, for example Light has no move and passes. If neither side can move the game ends, even when squares are still empty, and the discs are counted. A player who loses every disc also ends the game at once.
A move must turn over at least one of your opponent's discs. Only squares with a teal dot do that. A square next to an opponent disc is not enough: the line of their discs must end in one of yours, with no empty square between. In the opening, dark can play only d3, c4, f5 or e6.
No. To flip a disc you need your own disc on both ends of the line through it, and a corner is the end of every line that touches it. Once a corner disc is placed it keeps its colour for the rest of the game, which is why corners are worth taking.
At most 60. The board has 64 squares and four are filled at the start, and every move adds one disc. A game with passes or an early finish is shorter, for example when one side loses all its discs.
Not early in the game. Each disc you turn can give your opponent a new move, and the side with more moves usually controls the game. Easy plays this way and is the weakest level. Medium and Hard also count moves, corners and exposed discs, so grabbing discs against them tends to lose.
No. The game is processed in your browser and not uploaded. Your mode, level, colour, the game in progress and your win record against the computer are saved in this browser's local storage. The page counts visits with Google Analytics, such as that a game was started or won, never your moves.