Othello (Reversi) Intermediate: X-Squares, C-Squares, Stable Discs, and Parity

Once you understand why corners matter and what mobility is, the next step toward intermediate play is learning how not to hand your opponent a corner, and how to count the endgame. This guide covers X-squares, C-squares, stable discs, and parity (also called even theory).

X-squares and C-squares: don't give away corners

An X-square is the square diagonally next to a corner (b2, g2, b7, g7); a C-square is the edge square next to a corner (a2, b1, and so on). Playing an X-square or C-square early — the X-square especially — often lets your opponent take the adjacent corner, because your disc there becomes the hinge that flips straight into the corner. Until a corner is settled, treat X- and C-squares as moves of last resort: play them only after you have checked that you truly have nothing better.

Stable discs: build squares that can never flip

A stable disc is one that can never be flipped again for the rest of the game, no matter how either player moves. Corners are the seeds of stability: starting from a corner, your discs that connect along an edge become stable too. Because the result is decided only by how many discs you own at the very end, growing stable discs out from a corner converts directly into the final score. Discs you greedily flip in the midgame are not necessarily stable — they tend to flip back later.

Parity (even theory): play the last move

In the endgame, count the empty squares. When the board splits into separate empty regions, the side that plays the last move in a region is usually hard to flip there and tends to gain. Parity (even theory) is the idea of steering the odd/even count of empty squares so that you are the one who makes the final move. The exact counting is an advanced topic, but even just asking 'will I get the last move here?' in the endgame will change your results.

How to practice

These ideas are exactly the endgame reading that mate-in-N othello trains. TsumeDojo's tsume othello problems give you positions with a guaranteed winning line, every one verified by exhaustive search. Don't grab discs greedily — keep the corners, grow your stable discs, and count to the last square. Solve a few and the intermediate feel becomes second nature.