X-squares: the diagonal squares that hand over a corner

Published 22 October 2026 · By TG, the site operator

Our guide explains why a disc on a corner can never be flipped. This note is about the squares that give corners away. The square one step diagonally in from a corner is called an X-square: b2, g2, b7 and g7. The edge squares touching a corner, such as a2 and b1 beside a1, are C-squares. Both terms appear in an Othello glossary that the World Othello Federation featured in 2022.

Why the diagonal is the problem

Each X-square lies on a long diagonal running corner to corner through the centre, where the first four discs go. The official rules say every outflanked line must flip, and on this site diagonal lines count like any other. So a disc on b7 starts a path from a8 into the middle. When the squares beyond it hold your discs and then one of theirs, a8 is a legal move for your opponent, and it flips your X-square disc too.

The British Othello Federation's strategy pages, adapted from Brian Rose's book, call an early X-square move “almost certain to give up the adjacent corner”, though the book returns to some exceptions. Treat it as a strong habit, not a law.

An example checked on the site's engine

From the start, play F4, F5, E6, F3, C4, C6, G4, H5 (move-list names). Black, with 7 discs to White's 5, is to move. Row 8 is at the top; go by the names, since the board can be turned.

● Black, ○ White, · empty. Black: c4, d4, e4, f4, e5, f5, e6. White: c6, d5, h5, g4, f3.

Black has seven legal moves. b7 flips two discs, the others one each. The two X-squares among them are the only moves that let White into a corner:

• b7 flips c6 and d5, so b7, c6, d5 and e4 are Black with White's f3 behind them. White plays a8, flips all four, and Black's 10–3 lead becomes 6–8.
• g2 lines up the same diagonal from the other end. White answers h1, flips g2, f3 and e4, and again leads 8–6.

After d6, c5, h4, h3 or f2, White has no corner move at all. The greediest move is one of the two worst: two discs won, four handed back, and a White corner that stays White.

When the risk goes away

The danger lasts only while the corner is empty. In yesterday's puzzle the winning move was g7, an X-square, but h8 already held a White disc, so there was no corner to give away.

A C-square can open its corner only along its own edge, and the same chapter says its danger depends mostly on the rest of that edge: often worst when the neighbouring A-square (a3 next to a2, say) is empty, often best when your own discs already sit on the other squares of that edge. The guide's “giving the corner away” example is the bad case.

Using it on the site

Before you click a highlighted X-square, read the diagonal from its empty corner: if, after your move, it shows your discs and then one of your opponent's, the corner is theirs next turn. Against the computer, Undo lets you try both moves and compare. On Easy the computer picks a legal square at random, so it sometimes hands you a corner this way. On Normal and Hard its scoring counts every X-square disc against its owner and values a corner far above any other square. Start a game and watch the diagonals.

Sources

• 2: Corners & stable discs — British Othello Federation (adapted from Brian Rose's book)
• Othello glossary in Japanese, English, and Chinese — World Othello Federation, 25 August 2022
• オセロ用語対訳集 (Othello terms in Japanese, English and Chinese) — Othello glossary (English supervised by Brian Rose)
• Official rules for the game Othello — World Othello Federation

Written by the operator of this site from the sources listed above. Spotted a mistake? Email warmeve.label@gmail.com and it will be corrected.