Corners & Dangerous Squares

Why Corners Matter So Much

One caveat first: a corner is a very strong candidate, but that is not the same as an available corner always being the best move. At the end of this article are the figures from solving 400 positions exactly.

Othello discs flip again and again whenever they're flanked. But the four corners (a1, h1, a8, h8) can never be flanked from any direction. A corner you take stays your color until the end of the game. Discs like this are called stable discs.

Better still, a corner lets you build stable discs along the adjacent edges, so stable discs multiply from there. That's why Othello revolves around the fight for the corners.

Two Kinds of Dangerous Squares — X and C

If corners are strong, then the worst thing you can do is let your opponent take one. The squares most likely to gift a corner away are the two types right next to it.

X-squares (diagonally next to a corner)

The squares diagonally inside each corner — b2, g2, b7, g7 — are called "X-squares." Playing one early tends to open a path for your opponent to grab the neighboring corner, and it's very dangerous. As a rule, avoid X-squares in the opening and middlegame.

C-squares (along the edge next to a corner)

The squares along the edge next to each corner — a2, b1, g1, h2, and so on — are called "C-squares." C-squares can likewise hand a corner to your opponent as play develops. There are times they're fine, but as a beginner your safest instinct is to treat them with suspicion.

Make Your Opponent Play Next to the Corner

The advanced idea isn't to grab corners yourself — it's to create positions where your opponent is forced to play an X- or C-square. As you grind down their mobility (from the previous article), they run out of moves and get pushed into squares they don't want to play. The corner then falls into your hands.

Take a Corner, Then Extend the Edge

Once you own a corner, build discs along the edge from it. Edge discs connected to a corner become stable, letting you lock down the border in your color. Beware, though — an exposed disc in the middle of an edge (the shapes called a "mountain" or a "wing") can hand a corner back, so make sure the shape actually leads to stable discs.

Summary

• The four corners are stable discs that never flip. Check them first as a candidate.
• Don't play X-squares (b2, g2, b7, g7) early.
• Play C-squares (a2, b1, and friends) with care.
• Reduce your opponent's moves to force them next to a corner.

How Stable Discs Spread

Once you own a corner, you can build discs along the edges from it. Edge discs connected to a corner can no longer be flanked, so they become stable discs. Take a1, for example, and you can lock in a2, a3… and b1, c1… as they connect. A corner isn't "one square and done" — it's a launch point for growing stable discs. By contrast, an isolated disc in the middle of an edge (the "mountain" or "wing" shapes) tends to hand a corner to the opponent.

How X- and C-Squares Give Away a Corner

Why is an X-square (b2, g2, b7, g7) dangerous? Suppose a1 is still empty and you have a disc on b2: your opponent can use lines through b2 to reach a1, take the corner, and flip a whole swathe of your discs. C-squares (a2, b1, etc.) do the same over the next few moves. While a corner is empty, avoid the X- and C-squares around it.

If You Lose a Corner

Handing over a corner isn't game over. The key is to limit the damage: play to cut the opponent's edge connections so their stable discs don't multiply. Meanwhile aim for a different corner and keep your mobility edge, and you can recover in the endgame. Because only the final disc count decides the game, a single corner rarely settles it.

We Counted: Should You Always Take the Corner?

Related Articles

• Othello Basic Strategy — take fewer discs
• Endgame & Parity

Written and verified by MitchieRuns and edits Game DojoEditorial policy