What are X-squares and C-squares in Reversi?

The squares next to the corners have names because they lose so many games. Here is what X-squares and C-squares are, and why experienced players treat them like landmines.

Quick answer: X-squares are the four cells diagonally adjacent to the corners; C-squares are the cells directly beside a corner along an edge. Both are dangerous because playing them before the corner is settled usually hands your opponent a safe way to seize that corner. Avoiding premature X- and C-square moves is one of the first skills good players learn.

Where they are and why they hurt

Picture any corner. The square diagonally inward from it is the X-square; the two squares beside it along the edges are the C-squares. Playing an X-square early is the classic blunder: it typically lets your opponent play into the corner next to it and take it for free. Since corners are permanent, giving one away is often the whole game.

When you can play them

These squares are not forbidden forever - they become safe once the neighbouring corner is already yours or can no longer be taken. The real skill is manoeuvring so that your opponent is the one forced onto an X- or C-square, handing you the corner. Practise spotting them on the Othello board, where corner fights are central to the theory.

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Related questions

Why are corners so important in Reversi?

Corners are the most valuable squares in Reversi because a disc placed in a corner can never be flanked or flipped. It becomes a permanent anchor that lets you flip long lines for the rest of the game and stabilise entire edges. Most Reversi games are decided by who wins the corners.

How do you win at Reversi?

Winning Reversi comes down to three ideas: grab the corners safely because corner discs can never be flipped; play for mobility by keeping many good moves for yourself and few for your opponent; and control parity so you make the last move in each region. Chasing raw disc count early is usually a mistake.

Why do I keep losing at Reversi?

The most common reason players lose at Reversi is chasing disc count - building a big early lead that collapses when the opponent takes a corner and flips it back. The other frequent culprits are playing the dangerous squares next to empty corners and running out of good moves. Fix those three habits and your results jump.