Othello (Reversi) Glossary — Corners, Stable Discs, Mobility, Parity and the Key Endgame Terms

Othello (Reversi) commentary and endgame puzzles are full of special terms — stable discs, mobility, frontier, parity. Once you know what they mean, your understanding changes completely. Here are the most common terms in plain English; dip in wherever you like. The deeper thinking behind each topic is explored in the related columns at the bottom of the page.

Placing and flipping (the basic rule)

You play a disc of your colour so that it flanks one or more of the opponent's discs from both ends along a row, column, or diagonal; every flanked disc flips to your colour. You may only play where at least one disc flips — you cannot play a square that flips nothing. Whoever has more discs on the board at the end wins.

Legal move (a playable square)

A square you can actually play right now — one that flanks and flips at least one of the opponent's discs. In endgame Othello you choose, from among the legal moves, the single move that secures a win (locks in the best provable disc margin).

Pass

If you have no legal move at all, you pass and the turn returns to your opponent. You may not pass voluntarily when a move is available. When both players pass in a row (or all 64 squares are filled), the game ends.

Corner (the four corners)

The four corner squares. A corner has no outside square that could flank it, so once placed it can never be flipped — the strongest squares on the board. Winning or losing often hinges on who takes the corners, and securing a corner is frequently the core of the winning line in an endgame puzzle.

Edge

The outermost lines of the board (the border rows and columns). Edge discs are harder to flip than central ones and, when connected to a corner, readily become stable. But playing the edge carelessly can instead hand the opponent a corner (see X-squares and C-squares below).

X-square

The squares diagonally adjacent to a corner (b2, g2, b7, g7 in board coordinates). Playing one in the opening or middlegame is the classic dangerous move: it makes it easy for the opponent to take the neighbouring corner. As a rule, avoid them until you can clearly secure that corner.

C-squares, and A/B (the edge-square names)

Counting in from each corner, the edge squares are named C (right next to the corner), then A (the next one in), then B (the middle of the edge). Like an X-square, playing a C-square too early tends to invite the opponent into the corner. The A and B squares (nearer the middle of the edge) are comparatively safe and are good points when you do attack the edge.

Stable discs

Discs that are guaranteed never to be flipped again for the rest of the game. They grow outward from a corner, along edges and interiors that are completely filled with your discs. Steadily building stable discs is what becomes solid, unshakeable points in the endgame.

Frontier discs

Discs that touch at least one empty square. Only these frontier discs can be flanked and flipped by the opponent. The fewer frontier discs you have, the harder you are to flip and the fewer places your opponent has to play — so a small frontier is an advantage.

Mobility

The number of legal moves you have right now. The more choices you have (the higher your mobility), the better a move you can pick. Conversely, reducing the opponent's mobility — forcing them into a position where only bad squares are left — is a core Othello strategy.

Playing for fewer discs (disc count in the opening)

Having more discs in the opening and middlegame is not necessarily good. Taking too many discs increases your frontier, lowers your mobility, and makes it more likely you will be forced into a bad move. Since only the final count decides the game, deliberately keeping your disc count low along the way is standard practice.

Quiet move (interior move)

A move played inside an area already surrounded by your own discs, flipping only interior opponent discs. Because it creates almost no new frontier discs, it preserves your mobility and keeps your shape — a strong middlegame move. It flips few discs and looks modest, but it holds your mobility together.

Frontier count (openness theory)

The total number of empty squares adjacent to the discs a move flips. A move with a lower frontier count gives the opponent fewer new options and is generally better (openness/frontier theory). It is a handy yardstick for comparing middlegame moves; a quiet move is the classic lowest-frontier move.

Parity (the even/odd theory)

An endgame idea of looking at the remaining empty squares region by region and managing their parity (whether each region holds an odd or even number of empties). If you arrange to play the last move in each empty region, you flip a big batch and close it off in your favour. As a guideline, when the number of empty squares is even, the second player tends to get the last move.

The last move (tedomari)

Playing the final move in a given empty region in the endgame. Move last and the opponent cannot answer, so you flip a large number of discs and close the region on your terms — a big advantage. Parity is, in part, a way of making sure you are the one who gets this last move.

Perfect endgame reading (solving) and endgame Othello

Once few squares remain, you can read every line to the very end and choose the best (perfect play). TsumeDojo's endgame Othello puzzles are positions verified by exhaustive search to have a provable best result. They are practice in counting all the way to the final square, with corners, stable discs, and parity in mind.