Strategy
X-squares and C-squares
The short answer
X-squares are b2, g2, b7 and g7, the squares diagonally next to each corner. C-squares are the eight edge squares beside a corner, such as b1 and a2. Playing either one while that corner is empty usually lets your opponent take the corner on the following move.
Twelve squares on the board are worth less than nothing early in a game. Knowing which ones, and why, is the quickest single improvement available to a new player.
Where they are
Each corner has three neighbours: two on the edges and one on the diagonal. The diagonal neighbour is the X-square. The two edge neighbours are the C-squares.
| Corner | X-square | C-squares |
|---|---|---|
| a1 | b2 | b1, a2 |
| h1 | g2 | g1, h2 |
| a8 | b7 | a7, b8 |
| h8 | g7 | g8, h7 |
Why an X-square is so expensive
For your opponent to play a corner, they need a line of your discs running away from that corner with one of their own at the far end. The shortest such line is the diagonal, and the X-square is the first square on it.
Below, Black is to move and has 11 legal moves. One of them is g2. It flips 1 disc, which is the smallest flip available, and it looks like a harmless little developing move.
As soon as Black plays g2, White has h1 available and it flips 3 discs: g2, f3, e4. White will take it, and that disc is beyond reach for the rest of the game.
Why C-squares are the slower version of the same problem
A C-square sits on the edge next to a corner. Playing b1 does not hand over a1 immediately, because your opponent needs an unbroken line of your discs along the top edge with one of theirs beyond it, and at the start there is nowhere for that far disc to be.
The danger arrives when the C-square becomes the end of a run of your own discs. If you own b1, c1, d1 and e1 and your opponent gets a disc anywhere that lets them play a1, the whole row flips at once. That shape is called an unbalanced edge, and it is covered on the edge strategy page.
A useful working rule: a C-square is fine when it is isolated, and dangerous when it is attached to your own discs.
When an X-square is fine
- The corner is already taken. If you own a1, then b2 is just a normal square and often a good one, because it is protected. If your opponent owns a1, b2 costs you nothing extra.
- You have no choice. If it is your only legal move you play it, and the real mistake happened earlier.
- You have counted to the end. In the last dozen moves the exact disc count matters more than general rules, and sometimes a corner is simply not worth enough.
Being forced onto one
The worst version is not playing an X-square by choice, it is having nothing else. Below, Black has exactly 1 legal move in a real game, and it is b2. The score is 29 to 33, so the game is still close.
Black plays it, White answers a1, and that move flips 9 discs: b1, c1, d1, e1, f1, g1, b2, c3, d4. The game goes from 31 to 32 to a final score of 22 to 42.
Nobody blundered at move 59. Black lost the mobility fight twenty moves earlier and the X-square was just where the bill arrived. Mobility is where that game was actually decided.
Try it against the computer
Play the computer on Intermediate and set yourself one rule: never play b2, g2, b7 or g7 unless it is your only legal move. Most people find their results change immediately.
Next: corner strategy, edge strategy, stable discs, or back to the strategy guide.
Frequently asked questions
What is an X-square in Othello?
An X-square is one of the four squares sitting diagonally next to a corner: b2, g2, b7 and g7. Playing one while that corner is empty usually gives your opponent a diagonal line straight into the corner, so it is the most expensive routine mistake in the game.
What is a C-square in Othello?
A C-square is an edge square next to a corner. There are eight: b1 and a2 beside a1, g1 and h2 beside h1, a7 and b8 beside a8, and g8 and h7 beside h8. They are safer than X-squares but they still give the corner away when they sit at the end of a run of your own discs.
Is it ever right to play an X-square?
Yes, in three cases: the corner beside it is already taken, so nothing can be given away; you are forced because it is your only legal move; or you have counted the endgame and know the corner does not decide the result. Outside those, look for anything else first.
Why is b2 worse than b1?
b2 opens the diagonal a1-b2-c3, which is a straight line into the corner and needs only one of your discs on it for your opponent to take a1. b1 opens the top edge, which usually needs a longer unbroken run before a1 becomes legal, so the punishment is slower and often avoidable.