Reversi

Dark moves first. A move is only legal if it traps at least one of your opponent’s discs in a straight line — across, down or diagonally — between the disc you are placing and another of yours, with no gaps in between. Every trapped disc in every direction turns over, which is why a single move can change the whole board.

If you have no legal move you must pass and the turn goes back to your opponent. That is a normal part of the game, and this page says so on the status line whenever it happens. The game ends when neither side can move — usually but not always when the board is full — and whoever has more discs wins.

The playable squares are ringed while Show legal moves is on, and each square tells a screen reader what it holds and how many discs it would turn over, so the board can be played without seeing it.

Zerdly uses the standard preset: an eight by eight board, the four centre discs placed on their usual diagonals, dark to move first, and a pass when a player has no legal move. There are no house rules to toggle and no variant board sizes, so a result here means the same thing every time.

Undo takes back your last move — against the computer it also takes back the reply, since undoing one ply would simply hand you the same position. Because a pass is not a move, undo steps back until it is genuinely your turn again rather than counting plies. Show legal moves and Show coordinates are remembered between visits. Sound can be turned off at any time; nothing in the game is conveyed by sound alone. A game in progress is saved as you play and is waiting when you come back.

Versus the computer gives you six strengths. The weaker ones do not merely look less far ahead — they sometimes choose a move they can see is not the best, which is what makes them feel like an opponent rather than a machine with poor eyesight. The strongest never does, and from twelve empty squares it stops estimating and solves the rest of the game outright.

Two players, one device takes the computer out of it entirely. These games are counted but not scored: both players are here, so there is nobody for Zerdly to call the winner.

Analysis board has no opponent and no clock. You play both sides, step backwards and forwards along the line without losing it, and see what the engine thinks of every legal move — from the point of view of whoever is to play.

Tactical puzzles are twenty-four endings across three sizes. Each has been solved to the finish, so the answer is the move that produces the best possible final disc count rather than a matter of opinion, and it beats the next best by at least two discs. A wrong answer costs nothing, leaves the board untouched, and tells you what your move would have finished at next to what the best does.

Corners are the whole game. A disc in a corner can never be turned over, and it anchors a run along both edges that becomes permanent with it. That is also why the squares diagonally next to an empty corner are the most dangerous on the board: playing one usually hands the corner to your opponent.

Having more discs in the opening is close to meaningless, and often actively bad. What matters is how many moves you still have available and how few of your discs sit next to an empty square — a small, interior position is a strong one, and a board covered in your colour with nowhere left to play is a lost game.

Count the ending. From about ten empty squares the result is fixed by precise play rather than by judgement, and one move often swings the final count by a dozen discs. That is what the puzzles here are for.

The whole game runs in your browser. The opponent thinks on your own device and the puzzles are solved there too, both in background threads so the page stays responsive while they work — which matters here, because the strongest level can spend a few seconds on a move. No account is needed and nothing about your game is uploaded.

Your record, your saved game, your puzzle progress and your sound, motion and assist settings are stored in this browser only. Nothing here asks for a name or an email, and the Clear saved data button removes all of it — and only this game’s share of it.

Is it really free, and do I need an account?

Yes, and no. There is no sign-up, no trial and nothing to buy. The game runs in your browser and never sends your moves anywhere.

Why did my turn come straight back to me?

Because your opponent had no legal move, so they had to pass. That is a normal part of Reversi rather than a fault, and the status line says so whenever it happens.

Can I play against another person online?

No. Two-player here means two people at the same device, taking turns. There is no matchmaking and no online play, and nothing on this page pretends otherwise.

How strong is the computer?

The weakest level looks one move ahead and often plays something it can see is not best. The strongest looks six moves ahead, and once twelve squares are left it stops estimating and solves the rest of the game exactly — from there it cannot be outplayed.

Are the puzzle answers actually right?

Yes, and that is checked rather than assumed. Every puzzle is an ending solved to the finish, so the answer is the move producing the best final disc count — a fact, not an opinion. Each one also beats the runner-up by at least two discs.

What do the numbers on the analysis board mean?

The final disc difference each move leads to, or an estimate when the position is still too open to solve. They are always from the point of view of whoever is to move, and the panel says which side that is.

Where is my progress kept?

In this browser, and nowhere else. Clearing your browser data, or using the Clear saved data button on this page, removes it permanently.

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