Reversi X-Squares and C-Squares — The Danger Zones Explained

What Are X-Squares and C-Squares?

The 8×8 reversi board has 64 squares, but not all squares are created equal. The squares immediately surrounding each corner are classified into two danger categories:

X-Squares (4 squares)

The X-squares are the four squares diagonally adjacent to corners:

X-squares are widely considered the worst squares on the board in the opening and midgame. Playing on one almost always gives your opponent the adjacent corner.

C-Squares (8 squares)

The C-squares are the eight squares directly adjacent to corners along the edges:

C-squares are dangerous but less so than X-squares. They expose the edge approach to the corner rather than the more critical diagonal approach.

Together, X-squares and C-squares form the 12 “danger zone” squares — roughly 19% of the board that demands careful tactical awareness.

Why X-Squares Are So Dangerous

The reason X-squares are catastrophic is simple geometry. When you place a disc on an X-square, you create a diagonal outflanking line that leads directly to the corner.

The Basic Mechanism

Consider this scenario: you play a Black disc on b2. Your opponent now has a disc at b2 (it’s yours) and can place a White disc on a1 — the corner. The a1 placement outflanks your b2 disc along the diagonal, flipping it to White. But more importantly, your opponent now owns the corner permanently.

The problem is structural: the X-square sits on the only diagonal line that accesses the corner. By placing any disc on the X-square, you provide the stepping stone your opponent needs to reach that corner.

Why It’s Almost Always Bad

For your X-square play to not give away the corner, one of these conditions must hold:

1. You already own the adjacent corner (making the X-square safe)
2. Your opponent cannot make a corner play on their next turn (extremely rare before the endgame)
3. The game is ending in 1–2 moves anyway

In practice, conditions 2 and 3 rarely apply in the opening or midgame. Condition 1 is the main “safe X-square” scenario — but then you’re not really playing an exposed X-square, you’re reinforcing a corner you already own.

Why C-Squares Are Risky (But Not Fatal)

C-squares are less dangerous than X-squares for a structural reason: they expose the edge approach to the corner rather than the diagonal.

The Difference

When you play a C-square like a2, you give your opponent a potential path along column a toward corner a1. But this path requires your opponent to have a disc on the other side of a2 (at a3 or beyond) — which may not exist yet.

With X-squares, the diagonal is more likely to have the right disc alignment because the interior of the board fills up faster than the edges.

When C-Squares Are Acceptable

C-squares are often playable when:

• The adjacent X-square is empty. If you play b1 (C-square for a1) and nobody has played b2 (the X-square), the corner is reasonably safe because the diagonal isn’t exposed.
• You control the edge approach. If you already have discs along the edge beyond the C-square, you may be able to prevent the outflank.
• Late in the game. As the board fills, C-squares become standard plays that don’t carry the same risk.

When C-Squares Are Dangerous

C-squares become deadly when combined with X-square exposure:

• If both the X-square and a C-square near the same corner are occupied by your discs, the corner is extremely vulnerable.
• If your opponent has discs positioned to outflank along the edge through your C-square, they can both take the corner and flip your edge discs.

The 12 Danger Squares Mapped

Here’s how the danger zones relate to each corner:

Corner a1

Corner a8

Corner h1

Corner h8

When X-Squares Become Safe

Despite the general rule to avoid X-squares, there are specific situations where they’re acceptable:

1. You Already Own the Corner

If you have a disc on a1, playing b2 is completely safe. Your corner disc means b2 can never be used against you — it’s actually strengthening your corner position by potentially extending stability.

2. The Corner Is Already Taken (by Either Player)

If your opponent already owns a1, playing b2 doesn’t cost you anything because the corner is already gone. You might even benefit from the position if you can eventually flip the corner disc through other means.

3. Late Endgame (Last 5–8 Moves)

When only a handful of squares remain, X-squares may be the only legal moves available. At this point, the game is decided by parity and precise calculation rather than positional principles. X-squares in the final few moves don’t carry the same catastrophic risk.

4. Forced (No Alternative)

If the only legal move you have is on an X-square, you have no choice. This situation is itself a disaster — it usually means your mobility collapsed earlier due to positional mistakes. But when it happens, play it and try to recover.

How to Force Your Opponent onto Danger Squares

The flip side of avoiding X-squares and C-squares yourself is maneuvering your opponent onto them. This is the primary way to win corners.

Mobility Strangulation

The most reliable method: reduce your opponent’s legal moves until an X-square or C-square is their only option.

How:

1. Play quiet moves that flip 1–2 discs, keeping your frontier small
2. Focus on building interior discs in the center
3. Avoid expanding to the edges prematurely
4. Force your opponent’s available moves into a shrinking region of the board

When your opponent has only 2–3 legal moves and one of them is an X-square, they’re in trouble. If all of their legal moves are X-squares or C-squares, you’re about to win a corner.

Edge Manipulation

Sometimes you can create edge formations that force your opponent to play C-squares to maintain any presence on that side of the board. Wedges and unbalanced edges are the primary tools here.

The Patience Approach

Often, you don’t need to do anything aggressive. Keep making good central moves with high mobility, and eventually your opponent will run out of safe squares. X-squares and C-squares are what’s left when all the good moves are gone.

Common Mistakes with Danger Squares

1. Playing X-Squares “Because They Flip a Lot”

Beginners sometimes play b2 because it flips discs in multiple directions — it looks like a powerful move. The flip count is irrelevant. You just gave away a corner.

2. Playing C-Squares Without Checking the X-Square

Playing a2 feels safer than b2, and it can be — but only if b2 is still empty. If both a2 and b2 end up with your discs, corner a1 is hopelessly exposed.

3. Assuming C-Squares Are Always Safe

C-squares are less dangerous, not safe. In positions where your opponent has strong edge presence, a C-square play can give away the corner just as effectively as an X-square play — it just happens through the edge rather than the diagonal.

4. Not Recognizing Forced X-Square Situations

By the time you’re forced onto an X-square, the real mistake happened 5–10 moves ago when your mobility started collapsing. Learn to recognize shrinking mobility before you reach the danger square forced play.

For more errors like these, see the common mistakes guide.

Quick Reference

The golden rule: In the first 40 moves, avoid all 4 X-squares and be cautious with all 8 C-squares. Play in the center whenever possible. You’ll win more corners and more games.