The most important squares in Reversi, in order: corners, then edges adjacent to secured corners, then interior squares, then edges adjacent to empty corners (C-squares), and finally X-squares — which are the most dangerous positions on the board. This guide explains why each area has its value and how to use this knowledge in play. It pairs directly with the corner strategy guide and the stable discs guide.
The Classic Reversi Square Value Map
Reversi strategy researchers have assigned approximate static weights to the 64 squares. These numbers reflect how beneficial or dangerous each square tends to be across thousands of games:
a b c d e f g h
+----+----+----+----+----+----+----+----+
| 20 | -3 | 2 | 2 | 2 | 2 | -3 | 20 | 1
+----+----+----+----+----+----+----+----+
| -3 | -6 | -1 | -1 | -1 | -1 | -6 | -3 | 2
+----+----+----+----+----+----+----+----+
| 2 | -1 | 1 | 0 | 0 | 1 | -1 | 2 | 3
+----+----+----+----+----+----+----+----+
| 2 | -1 | 0 | 0 | 0 | 0 | -1 | 2 | 4
+----+----+----+----+----+----+----+----+
| 2 | -1 | 0 | 0 | 0 | 0 | -1 | 2 | 5
+----+----+----+----+----+----+----+----+
| 2 | -1 | 1 | 0 | 0 | 1 | -1 | 2 | 6
+----+----+----+----+----+----+----+----+
| -3 | -6 | -1 | -1 | -1 | -1 | -6 | -3 | 7
+----+----+----+----+----+----+----+----+
| 20 | -3 | 2 | 2 | 2 | 2 | -3 | 20 | 8
+----+----+----+----+----+----+----+----+
The four zones that matter most:
| Zone | Squares | Typical Weight | Why |
|---|---|---|---|
| Corners | a1, a8, h1, h8 | +20 | Can never be flipped; anchor stable edges |
| X-squares | b2, b7, g2, g7 | −6 | Almost always give opponent corner access |
| C-squares | a2, b1, a7, b8, g1, h2, g8, h7 | −3 | Risk exposing corners |
| Stable edges | Other edge squares near secured corners | +2 | Become unflippable once corner is taken |
| Interior centre | c3–f6 (excluding diagonals) | 0–1 | Flexible; support mobility |
Important: These are static weights — starting approximations, not absolute rules. Dynamic factors (corner ownership, mobility, game phase) override them in real play. For a full strategic framework, see the strategy guide.
Why Corners Dominate: The Math
A corner disc has a unique geometric property: it borders only two squares, both of which are adjacent squares, not bracketing positions. To flip a disc, you need it to be between two opponent discs in a straight line. A corner disc can never be in the middle of any line — it’s always at the end.
The cascade effect of a corner:
- Corner taken → disc is permanent
- You play along the edge from the corner → that edge disc is now stable (has a corner backing it and a wall on one side)
- You extend further along the edge → entire edge row becomes stable
- Edge discs force your opponent away from that region
- You can now threaten the adjacent corner using your edge control
This cascade is why corners are worth roughly 20× a regular disc in static valuations. In practice, a single corner lead can determine the game.
X-Squares: The Most Dangerous Positions
X-squares (b2, b7, g2, g7) are diagonally adjacent to corners. They receive the highest negative weight (−6) for one reason: playing on an X-square when the adjacent corner is empty almost always gives your opponent a path to that corner.
Why X-squares are dangerous:
When you play b2, you create a situation where your opponent can potentially play a1 (the corner) on a future move, placing their disc next to your b2 disc in the diagonal direction. Your b2 disc becomes part of the line they need for the corner.
When X-squares are acceptable:
- The adjacent corner is already taken (by either player)
- You have a forced win that requires the X-square move
- You can take the corner yourself on the very next move
- You’re in the endgame and all other moves are provably worse
Outside of these specific situations, treat X-squares as forbidden territory.
C-Squares: Handle With Caution
C-squares are the edge squares directly adjacent to corners:
- a2, b1 (adjacent to a1)
- a7, b8 (adjacent to a8)
- g1, h2 (adjacent to h1)
- g8, h7 (adjacent to h8)
C-squares get a negative value (−3) because they can set up your opponent to play into the corner via the adjacent X-square, or sometimes directly.
When C-squares become safe:
- The adjacent corner is already taken by you — now the C-square is part of a stable edge
- The adjacent corner is taken by your opponent — the C-square has different dynamics; it may connect to opponent stability
- You can guarantee taking the corner immediately after
Dynamic vs. Static Values
Static weights like the table above are useful starting points but have significant limitations:
How Phase Changes Values
Opening (moves 1–20): Interior squares matter most. Corner access is the priority concern. The static values roughly apply.
Midgame (moves 20–44): Corner control is increasingly decisive. C-squares near secured corners become strong positives. The X-square penalty remains — never play them against an empty corner.
Endgame (moves 44–60): All static values become secondary to exact disc counting. A disc in the “worst” position (like c3) is worth exactly 1 disc in the final count. The only endgame geometry that matters is parity and which player takes the last move in each region.
Mobility Overrides Position
A common beginner mistake is rigidly following the square value map while ignoring mobility. A high-value square played at the wrong time can:
- Give your opponent 8 new available moves
- Reduce your own mobility to 2 moves
- Force you into playing X-squares or C-squares on your next turn
Always check: how many moves will I have after this play? How many moves will my opponent have? A move that gives you 5 options and your opponent 2 is usually better than a move that hits a +2 square but gives your opponent 8 options.
Practical Application: Reading the Board
When looking at a position, go through this mental checklist:
- Are any corners available? If yes, take one. Always.
- Does any move give my opponent corner access? If yes, only play it if all alternatives are worse.
- After my candidate move, how many moves do I have? How many does my opponent have?
- Am I creating frontier discs? Moves that surround your opponent’s discs (internal positions) are better than moves that expose your discs to the edges.
- Is it endgame (≤20 empty squares)? Switch to exact counting — the map no longer applies.
Board Value Maps in Reversi AI
Most Reversi AI programs use board value maps as one component of their position evaluation function. However, strong AI combines static square weights with:
- Mobility scoring — Difference in legal moves available to each player
- Stable disc counting — How many discs of each color can never be flipped
- Frontier disc counting — Discs adjacent to empty squares for each player
- Pattern recognition — Learned weights from thousands of analyzed games
- Exact endgame solving — Perfect calculation in the final 20 moves
This is why simply occupying high-value squares isn’t enough to beat strong AI. The AI weights these factors dynamically. Understanding why squares have the values they do — not just memorizing the map — is what lets you adapt to any position.