Why Corners Matter

The four corner squares — a1, a8, h1, h8 — are the most powerful positions in reversi. A corner disc is permanently stable: once placed, it can never be flipped for the rest of the game. No opponent can place a disc beyond it because there are no squares past the corner in any direction.

This permanence is unique. Every other disc on the board can potentially be flipped — even discs that have sat untouched for 30 moves. But corners are forever. In a game where the board constantly shifts and pieces change sides, corners are the only truly safe ground.

But the power of corners extends far beyond a single stable disc. Corners cascade — they make adjacent discs stable, which makes their neighbors stable, which builds walls of permanent territory that your opponent can never reclaim.

Statistical reality supports this: in competitive play, the player who controls more corners wins the majority of games. Not all games — position and skill matter enormously — but corners tilt the odds significantly.

How Corner Stability Cascades

When you take a corner, it doesn’t just protect that one square. It triggers a chain of stability along the connected edges.

The Edge Cascade

Consider taking corner a1. Now any disc you place on a2 is also stable — because the only way to outflank a disc on a2 from the a-column direction is from beyond a1, and there’s nothing there. Similarly, b1 becomes stable because it’s anchored to a1 along row 1.

As you fill in more of the edge, the stability cascades:

  • a1 → a2 → a3 → … (along column a)
  • a1 → b1 → c1 → … (along row 1)

If you own both ends of an edge — say a1 and h1 — the entire top row becomes stable once filled. That’s 8 permanently locked discs from two corner investments.

The Diagonal Cascade

Stability also cascades diagonally from corners, though more slowly. If you own a1 and have filled the a-column and row-1 edge, disc b2 becomes stable because it’s locked in from three sides. This cascading effect can extend deep into the board in late-game positions.

For a complete treatment of how stability propagates, see stable discs explained.

The Four-Corner Endgame

In the endgame, the player with 3 or 4 corners has an overwhelming advantage. With all four corners secured, you can build stable walls on all four edges, often locking up 24–32 permanent discs before the final sequence even begins.

Even 2 corners on the same edge is extremely powerful — it guarantees the entire connecting row becomes stable once filled.

Corner count and win rates:

Corners Won Approximate Win Rate
4–0 ~95%+
3–1 ~80%+
2–2 (same edge) Slight advantage to the player with the edge connection
2–2 (diagonal) Roughly even, depends on position
1–3 ~20% or less
0–4 ~5% or less

These are approximate figures from competitive play. Position, parity, and remaining mobility modify these numbers significantly. But the trend is clear: corners are the best predictor of who wins.

How to Win Corners

Corners don’t just appear — you have to create the conditions for taking them. Here are the primary methods:

1. Force X-Square Plays

The most common way to win a corner is to force your opponent onto the adjacent X-square. If your opponent plays b2, you can almost always take a1 on your next move.

How to force X-square plays:

  • Limit their mobility. When your opponent has few legal moves, they may be forced to play on X-squares because nothing better is available.
  • Control the center. A player with dominant center control forces the opponent toward the edges and danger squares.
  • Use quiet moves. Quiet moves that flip 1–2 discs keep your frontier low, which reduces your opponent’s legal moves and pushes them toward bad squares.

2. Edge Pressure

Building a presence near an edge can create situations where your opponent must play C-squares or X-squares to maintain any legal moves in that sector. See edge strategy for wedges and unbalanced edge techniques.

3. Tempo Forcing

Sometimes you can create a sequence of moves where each of your plays forces a specific response, ultimately culminating in a position where the only remaining move for your opponent gives you a corner.

4. The Waiting Game

Sometimes the best way to get a corner is to do nothing aggressive — just make quiet, central moves that maintain your mobility while your opponent runs out of safe squares. Eventually, the board fills up and the only remaining legal moves are on danger squares.

When to Sacrifice a Corner

In the vast majority of positions, taking a corner is correct. But advanced players recognize rare situations where a corner sacrifice is justified:

1. Parity Trade

In the late endgame, taking a corner might cost you parity — the ability to play last in key regions. If sacrificing corner access lets you make the final move in a critical region, the parity advantage can outweigh the corner’s value.

2. Corner for Corner Trade

If taking one corner immediately gives your opponent a different corner (via a flipping chain that exposes an X-square elsewhere), the trade may not be beneficial. In these cases, evaluate which corner creates more stable territory.

3. Tempo and Mobility Collapse

Occasionally, taking a corner would collapse your mobility by forcing you to flip a large number of discs, giving your opponent a completely free board with unlimited options. The mobility loss can outweigh the corner gain — though this happens only in sophisticated endgame positions.

For Beginners: Always Take the Corner

Unless you’re a strong intermediate or competitive player, don’t worry about exceptions. Take every corner that’s available. The situations where declining a corner is correct are extremely rare and require deep calculation that develops only with significant experience.

The Corner Sacrifice Trap

A crafty opponent may deliberately give you a corner if doing so puts you in a worse overall position. For example, if taking the corner forces you to flip discs that give them access to the other three corners, the “gift” corner was actually a trap.

Defense: Before taking a corner, quickly check what it exposes. Does taking a1 flip discs that give your opponent access to a8, h1, or h8? If so, you might still take it — but be aware of the consequences.

The Double-Corner Edge

If you own corners a1 and h1, and your opponent has no discs on row 1, you can gradually fill the entire top row with your discs — every single one will be stable. This is one of the most dominant positions in reversi.

The Corner Race

When both players are maneuvering for the same corner, the player with more mobility usually wins. If you have 8 legal moves and your opponent has 3, you can afford to wait while they’re forced into bad plays. This is why mobility strategy and corner strategy are deeply intertwined.

Corner Strategy Summary

Principle Application
Corners can never be flipped Always take them when available
Corners cascade into edge stability Build walls after securing corners
X-squares give away corners Avoid X-squares; force opponent onto them
More corners ≈ higher win rate Corner count is the strongest win predictor
Corner sacrifice is rarely correct Beginners should always take the corner
Mobility enables corner access High mobility lets you wait for corner opportunities

What to Study Next

Corner strategy connects directly to several other concepts: