In Reversi the disc count in the middle of the game means almost nothing; corners and the number of moves you leave your opponent decide it. One rule set does most of the work: take a corner whenever you can, never play next to an empty corner, and otherwise choose the move that leaves the computer the fewest replies. In simulation that wins 97% of games against Easy and 89% against Hard here, and both of those levels can also be beaten by a fixed list of moves, the same every time.
Every disc on the board can still change colour, usually more than once, so a lead in discs is a lead in things your opponent can flip. Worse, a big group of discs gives the other side plenty of places to play, while a small one leaves it short of moves.
Easy shows this most plainly, because it always flips as many discs as it can. Against a player who just takes corners and stays off the squares next to empty corners, Easy is ahead on discs after 30 moves in 98.6% of games — and loses 88% of them.
Between two equal players the count says just as little. In 10,000 simulated games between two copies of a decent player, the side ahead on discs after 30 moves won only 45% of the time — a coin toss, if anything tilted against the leader. Over the last ten moves of a game the score moves by 16 discs in a typical game and by 36 or more in one game in ten, and the side leading ten moves from the end went on to lose a third of the time.
The “decent player” here is the page's own Expert search cut to three moves ahead, with one move in ten played at random so that games differ.
A corner disc can never be flipped: there is no square beyond it on any line. It also anchors the edges next to it, because discs that grow out of a corner along an edge become unflippable too. In the same 10,000 games, whoever took the first corner won 75% of them, and the side that ended with more corners won 88%.
So the squares that hand a corner over are the ones to fear. There are two kinds next to each corner:
Against Expert the price is high. In our games, when a disc landed on an X-square while its corner was empty, Expert ended up owning that corner 79% of the time, and in 39% of all cases it took it within three moves. And Expert does not wait for you to make the mistake: it plays so that you run out of other moves.
Both rules are about empty corners. Once a corner is taken, the squares beside it are ordinary squares, and often good ones for whoever owns the corner.
A move in Reversi is only legal if it flips something, so the number of moves you have depends on the shape of the position, not on how many discs you own. The player who runs short of moves has to take whatever is left — a square next to a corner, a move that opens an edge — or passes and lets the opponent move twice.
Three habits keep your options open and take the opponent's away:
The effect is easy to measure. Adding just the last habit to the corner rules — corners first, never next to an empty corner, then the move that leaves the computer the fewest replies — raises the win rate against Hard from 72% to 89%. In those games the side with more legal moves after 30 moves won 89% of the time, and the side with fewer frontier discs won 90%.
Mobility is also how Expert wins. Against it, a player who follows the two corner rules was left at some point with nothing but moves next to empty corners in 99.9% of games, typically with 32 squares still empty — half the game still to play. Against Hard that happened in 38% of games, and only in the last few moves.
Edges are where stable discs are made. A run of discs along an edge that starts from your corner cannot be flipped, and every move added to it is permanent. An edge without a corner is a different matter: a lone disc in the middle of an edge can be outflanked along the edge itself, and the edge squares next to it may later leave you with nothing but a C-square to play.
The endgame is where Reversi changes hands. With few empty squares left, every move flips lines in several directions at once, and passes become common: a player with no legal move simply watches the opponent play twice. That is why a disc lead with ten moves to go is worth so little.
Expert switches to a plain disc count once twelve or fewer squares are empty, and on a fast device, with about seven empty squares left, its search reaches the end of the game. Expect it to play the last moves very accurately; the game against it has to be won before that.
Here is how simple ways of choosing a move do against each level (you play blue and move first; ties between equally good moves broken at random):
| Your rule | vs Easy | vs Hard | vs Expert* |
|---|---|---|---|
| A random legal move | 36% | 15% | 0.1% |
| Flip as many discs as possible | 50% | 17% | 0% |
| Play the square with the best value in the classic weight table | 85% | 44% | 2.5% |
| Take a corner when you can, otherwise a random move | 65% | 34% | – |
| Corners first, never next to an empty corner, otherwise random | 88% | 72% | 1.1% |
| Corners first, never next to an empty corner, then the fewest replies for the computer | 97% | 89% | 0.2% |
* Expert is limited by time, so how deep it looks depends on the device. The Expert column was simulated at a fixed depth of five moves: 3,000 games for each corner rule, 1,000 for the others. On a fast computer it usually reaches seven. A check at depth six gave the same picture: the best corner rule won 1.9% of 1,000 games. A dash means not simulated.
Easy and Hard involve no chance at all: the same position always gets the same reply. So a game that beats them once beats them every time, and the shortest ones are short. Squares are named as on a chessboard, letters a–h from left to right and rows 1–8 from the top (the game itself shows no labels); you start with e4 and d5, the computer with d4 and e5.
Easy, in eight moves. Play c4, c6, e6, a4, f4, e2, g4, h5. The computer answers c5, b5, f5, e3, f3, b4, g5, and your eighth move leaves it with no discs at all: 19:0. No shorter win exists — we checked every line of seven moves or fewer.
Hard, in seventeen moves. Play d3, b3, c4, a1, a2, c2, d2, a4, e1, e2, b4, f2, h1, b5, a6, b6, f5. Hard replies c3, b2, a3, e3, c1, c5, d1, b1, then passes after e1, answers e2 with f1, passes after b4, answers f2 with g1, passes after h1, answers b5 with a5, passes after a6 and b6, and f5 ends the game 32:0. Hard counts only discs, four moves ahead, so it happily grabs material while you take the corners and the whole top edge; by the time it runs out of moves, it is too late. No win of seven moves or fewer exists against it.
Expert has no such recipe: how deep it searches depends on how fast your device is, so the same moves can get different answers. Against it, none of the rules in the table wins as many as three games in a hundred. It plays by the same principles as this page, only several moves deep, and it wins mostly by leaving you without safe moves halfway through the game: the player following our best rule was ahead on discs after 30 moves in 74% of games against it and still won only 0.2%. A win over Expert takes the same ideas applied further ahead — counting its replies to your move and your replies to its answer, not just the next move.
The first two rules alone beat Easy in 88% of games and Hard in 72%; the third takes those to 97% and 89%.
Take corners, avoid the squares next to empty corners, and play for mobility: of your safe moves, choose the one that leaves the opponent the fewest replies. Against the computer here those three rules win 97% of games on Easy and 89% on Hard in simulation.
Not before the end. Between equal players, the side ahead on discs after 30 moves wins only about 45% of games, and against Easy a player who follows the corner rules is behind on discs in 98.6% of games at that point and still wins 88%.
A disc on b2, g2, b7 or g7 while the corner beside it is empty usually gives the corner away. Against Expert here, a disc on an X-square beside an empty corner cost that corner in 79% of cases. Once the corner is occupied, the square is safe.
Yes. Easy and Hard are fully predictable, and fixed lines of moves beat them every time: eight moves wipe Easy off the board (19:0) and seventeen do the same to Hard (32:0). Expert is much stronger: none of the simple rules on this page wins as many as three games in a hundred against it, so beating it takes the same ideas calculated several moves ahead.
Wiping out the opponent: when one side has no discs left, neither can move and the game is over. Against Easy here that takes eight moves (c4, c6, e6, a4, f4, e2, g4, h5), which is the shortest possible against it.
On the standard 8×8 board, a computer analysis published in 2023 concluded that perfect play by both sides ends in a draw. In practice, games between people and programs are decided long before that question arises, by corners and mobility.
Practically. That name belongs to a trademarked version of the 19th-century game, settled in Japan in 1971, with the four starting discs fixed in the centre. The game here starts the same way and follows the same rules.