Play Reversi online, free, against the computer

How this Reversi game works

The board starts with four discs in the middle. You place a disc so that it traps a straight line of the computer's discs between your new disc and another of yours, and every trapped disc flips to your colour. Legal squares glow on your turn, flipping is animated so you can see exactly what changed, and if a side has no legal move it passes automatically. The game ends when neither side can move, and the final count decides it.

There are Easy and Hard settings, a new game button, and tap controls for phones.

Where it comes from

Reversi appeared in England in 1883, credited to Lewis Waterman and John W. Mollett, who fell out over who invented it. Nearly a century later a Japanese variant with a fixed starting position took the game worldwide, and that is the version most tournaments use. Our rules match that familiar four-disc start.

The counter-intuitive strategy

New players try to flip as many discs as possible. Strong players do the opposite for most of the game. Having fewer discs mid-game often means having more choices, and choices are what win.

• Corners are gold. A disc in a corner can never be flipped, and it anchors the edges next to it.
• Don't give corners away. The squares diagonally next to a corner are dangerous, because they can let your opponent take it.
• Think mobility. Count your legal moves against the computer's. Try to leave it with only bad ones.
• Edges help once you control a corner, and hurt when you don't.

Common mistakes

• Racing for the edges. Edge discs are useful once you hold a corner, and a liability when you don't.
• Taking the squares next to a corner. Those squares often hand the corner to your opponent.
• Counting discs too soon. The disc count only matters at the end, and the player ahead early is often the one about to run out of safe moves.

The endgame

With about ten squares left, start counting parity: who gets the last move in each empty region? Getting the final move in a region lets you flip a lot of discs with no reply. A move that looks small can swing the result, so slow down and count the number of moves each side has left in each pocket of empty squares before committing.

Made from a prompt

The game started as a written brief to an AI coding assistant. The prompt is on the game page, including the instruction not to borrow any trademarked name.