Reversi Guide

Reversi: Nobody Knows Until the Last Move Lands

Reversi's whole appeal is the comeback. Even if the board looks completely covered in your color, your opponent taking just one corner properly can flip the entire game within a few moves. Players alternate placing discs on an 8×8 board, flipping any opponent discs sandwiched in a line between two of their own, and whoever has more discs at the end wins. It dates to 19th-century England and was popularized again worldwide in the 1970s under the trademarked name "Othello."

There's exactly one thing separating beginners from strong players: it's not who flips the most discs, it's who claims the right squares.

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Objective & Rules

• Four discs start diagonally placed in the center (2 black, 2 white), and Black moves first.
• You may only play where opponent discs are sandwiched in a straight line between your new disc and an existing one of yours, and every sandwiched disc flips to your color.
• A legal move must flip at least one disc. If you have no legal move, your turn passes automatically. If neither player can move, the game ends.
• Whoever has more discs when the board is finished wins.

Corners: The Only Squares That Never Flip

The four corner squares, once claimed, can never be flipped by any sequence of moves. They're permanently stable. A single corner is usually worth more than several ordinary squares combined, and claiming one also helps stabilize the edge discs extending from it. Much of a Reversi game boils down to a simple question: who claims the corners, and how many.

Dangerous Squares: Handing Away a Corner for Free

• X-squares: diagonally adjacent to a corner. Playing here first often lets your opponent flip it and take the corner along with it. It's the single most common beginner mistake.
• C-squares: edge squares next to a corner. Not quite as risky as X-squares, but they still tend to give the corner away.
• The basic rule: while a corner is still empty, leave its neighboring X and C squares alone.

Mobility: The Paradox of Flipping Less

Beginners love flipping as many discs as possible in one move. Strong players do the opposite. Flipping more exposes more of your discs across the board, which in turn gives your opponent more legal moves, in other words more mobility. Flip less, and your exposure shrinks while your opponent's options shrink too, leaving you with more squares to choose from and more freedom to pick a favorable one. Expanding your own mobility while narrowing your opponent's is essentially the entire midgame.

Opening, Middle, Endgame: Different Goals at Each Stage

• Opening: don't get greedy. Stay near the center, flip as little as possible, and keep a tidy, stable shape.
• Middle: calculate your route to the corners, avoid X and C squares, and steer your opponent into taking the bad ones instead.
• Endgame: count the remaining empty squares and calculate the final flips precisely, because a single late move can flip dozens of discs at once.

How Omokboard's Reversi AI Plays

In AI games you play Black and move first, and the AI plays White.

• Easy picks one of its legal moves completely at random. It often walks right past an open corner, which makes it a good partner for learning where the corners, X-squares, and C-squares are.
• Normal looks only one move ahead. It scores each legal move by adding the square's position value, the number of discs flipped, and mobility after the move (how many moves it would have minus how many you would have), then picks at random from the top three.
• Hard reads four moves ahead, alternating its moves and yours. Once 8 or fewer empty squares remain, it reads all the way to the end and finds the move that leaves it with the most discs.

The position table the AI uses to evaluate the board puts this guide's principles into numbers. A corner is worth +120, an X-square -45, a C-square -25, and the edge square two steps from a corner +20. Hard also checks one more thing: once a corner is occupied, it drops the penalty for the X- and C-squares next to it. After someone owns that corner, those squares no longer hand it away.

In 200-game AI-vs-AI matches, Hard beat both Normal and Easy in over 99% of games, and Normal beat Easy about 87% of the time. Because Normal looks only one move ahead, it often plays an ordinary square that still opens a path to a corner for you. Check whether a corner has opened up after every AI move, and Normal is very beatable. Hard makes that mistake far less often. Its evaluation only covers position values and mobility, though, so it never counts stable discs, the ones that can never be flipped. Edge battles that are decided more than four moves out are the place to look for an edge against Hard.

Common Mistakes

• Getting greedy early and flipping too much, leaving discs overexposed across the board.
• Playing an X or C square while the corner next to it is still empty, effectively handing it over.
• Reacting emotionally to the mid-game score, when comebacks in Reversi are common, not rare.

Strategy by Skill Level

Just starting out? Memorize where the corners and X/C squares sit. That one habit alone eliminates most of the embarrassing losses beginners run into.

Getting comfortable? Consciously flip fewer discs per move. It can feel like giving something up in the moment, but you'll start to notice the mobility advantage pay off.

Want to go further? Practice counting the endgame. Once fewer than about ten squares remain, actually calculating several moves ahead, instead of relying on feel, makes a real difference to your win rate.

FAQ

• Are Reversi and Othello the same game? Nearly identical. Othello is a trademarked name for a standardized starting position and rule set.
• Can I come back from way behind? Yes. Corners and precise endgame counting frequently flip the result dramatically in just one or two moves.
• What if I have no legal move? You pass automatically and your opponent continues.
• How far ahead does the AI think? Easy plays at random and Normal looks just one move ahead. Hard reads four moves ahead, and once 8 or fewer empty squares remain, it reads all the way to the end. Start on a lower difficulty to build a feel for corners and X/C squares first.