Reversi

How do you play Reversi?

Reversi is played with discs that are black on one side and white on the other. Each player owns one colour. The board starts with four discs in the centre, two of each colour, placed diagonally. Black moves first, and then the players alternate turns.

On your turn you place one disc of your colour on an empty square. The move is only legal if it flanks at least one line of your opponent's discs between the new disc and another disc of yours. Every flanked line is flipped to your colour. When the board is full, or neither player can move, the player with more discs wins.

In our game, small dots mark every legal square for the player to move, so you never have to guess. Tap a dot to play, watch the discs flip, and tap the hint button when you want the engine's suggestion for the best move.

What are the full Reversi rules?

A legal move must outflank. Starting from the square you choose, look in any of the eight directions: left, right, up, down and the four diagonals. If you see one or more opponent discs in a straight unbroken line, and the line ends with one of your own discs, all those opponent discs flip. One move can flip in several directions at once.

Only discs in the lines created by your new disc flip. There are no chain reactions, so flipped discs do not trigger more flips. If you have no legal move, you pass and your opponent moves again. If neither player has a legal move, the game ends even if squares remain empty. You must move whenever you can, and passing is never a choice.

• Board sizes: 6x6 (36 squares), 8x8 (64 squares) and 10x10 (100 squares).
• Black always moves first.
• A move must flip at least one disc.
• No legal move means a forced pass.
• Most discs at the end wins, equal counts are a draw.

What is Reversi and where does it come from?

Reversi is an English board game from the 1880s, and the version on an 8x8 board with a fixed starting position became hugely popular in the twentieth century. It is also sold as Othello under a brand name, and the rules are the same apart from the traditional detail that older Reversi sets let players choose their first four moves. Here we use the modern fixed start, which is what almost every online player expects.

The game is famous for its slogan, a minute to learn and a lifetime to master. The rules fit in a paragraph, yet the strategy is deep. A big disc lead in the middle of the game is usually a warning sign, not a good position, and that surprises many beginners.

What is the best Reversi strategy?

Beginners try to flip as many discs as possible on every move. Strong players do almost the opposite. In Reversi, the disc count only matters at the very end, and the real fight is for control of the moves. The ideas that matter most are corners, mobility, stable discs, frontier discs and parity.

Use this order of priorities as a simple plan. Take a corner when it is safe. Avoid giving your opponent a corner. Prefer moves that leave you more options and your opponent fewer. Flip few discs and keep them inside the pack. Then, in the last moves, count carefully and play for parity.

• Corners cannot be flipped, so they anchor stable regions.
• Mobility is the number of legal moves you have compared to your opponent.
• Frontier discs touch empty squares and give your opponent new moves.
• Parity is about getting the last move in each empty region.

Why do corners matter in Reversi?

A disc in a corner can never be flipped, because a line cannot extend past the edge of the board on both sides. From a corner you can build chains of stable discs along both edges, and these chains are permanent. Corners also give you safe moves later in the game, which helps when other moves would hurt you.

The corner strategy is simple to say and harder to use: take corners, and never hand them over. The best way to win a corner is to make your opponent run out of safe moves, so they are forced to play next to it.

What are X-squares and C-squares?

The X-squares are the four squares diagonally next to each corner. The C-squares are the squares next to a corner along the edge, two beside each corner. Playing on either while the corner is empty is risky, because it often lets your opponent capture the corner by flanking along the diagonal or the edge.

The X-square is the most dangerous of all, and you should avoid it unless the corner is already taken or you have worked out that the opponent cannot use it. C-squares are less dangerous, especially once the edge is partly filled, but treat them with caution. If you have the choice between a quiet interior move and an X-square, choose the quiet move.

What is mobility in Reversi?

Mobility is the number of moves you can make, compared with your opponent. High mobility means you have choice and your opponent is cramped. A player who has only one or two legal moves can be forced into a bad square, such as an X-square, which gives away a corner.

To keep mobility, avoid walls. A wall is a long line of your discs on the outside of the position, because every empty square next to it becomes a move for your opponent. Prefer quiet moves that flip discs in the middle of the board, so you do not open new squares for the other side. This is why having fewer discs in the opening and middle game is so often a good thing.

What are stable discs and frontier discs?

A stable disc is one that can never be flipped for the rest of the game. Corners are always stable, and discs connected to a corner along a filled edge are stable too. The more stable discs you have, the more of your final score is safe.

A frontier disc is one that touches an empty square. Frontier discs help your opponent, because each empty neighbour can become a legal move for them. Interior discs, surrounded by other discs, give no such help. Aim for fewer frontier discs than your opponent, and your mobility usually follows.

What is parity in Reversi?

Parity is about the end of the game. Empty squares often form separate regions. If a region has an even number of empty squares, the player who moves second in it usually gets the last move there, and if it has an odd number, the player who moves first does. The last move in a region is valuable, because it cannot be answered by a flip.

In practice, try to leave odd-sized regions for yourself to start in, and avoid being the first to move into an even-sized region. Black moves first on the board, and with an even number of squares, White usually gets the last move overall, so Black needs to win parity fights in the regions. Parity is a tie-breaker, not a first rule, so only use it when other factors are equal. A good player counts empty squares in each region during the last twelve moves or so.

How do you beat the computer at Reversi?

Start on Easy to learn the shape of the game, then move to Medium and Hard. Easy and Medium make plenty of mistakes, often because they like flipping many discs. Hard uses alpha-beta search with positional weights, mobility and corner parity, and in the last eleven or so empties it solves the endgame exactly, so it will not slip up near the finish.

To beat Hard, you need a plan. Play quiet moves, keep your frontier small, and wait for it to run short of safe moves. Use the undo button to test an idea, and the hint button to compare your choice with the engine's. After a loss, replay the game and look for the move where your mobility dropped. That is usually the turning point.

Which board is best: 6x6, 8x8 or 10x10?

The 8x8 board is the standard, and most strategy advice is written for it. It was shown in 2023 that perfect play on 8x8 is a draw, though no human or program on a phone plays perfectly. The 6x6 board is smaller and quicker, and has long been known to be a win for the second player, White, by a small margin with perfect play. It is a nice size for a short game on a phone.

The 10x10 board is larger, with more squares and a longer middle game, so planning matters more and a single mistake is easier to recover from. Try 6x6 to learn, 8x8 to compete, and 10x10 when you want a long game against a friend.

Can you play Reversi with friends online?

Yes. Choose the online mode, create a room, and send the room link to your friend. They open it on their phone or computer and you are playing, with no account or download. If you are in the same room, use the two-player mode and pass the screen between turns, which is handy on a train or at a table.

Reversi is free to play on PlayNest, and it works on phones, tablets and computers in the browser. Your game is saved so you can come back to it, and the board has large touch targets, so it is comfortable on a small screen.