What is the difference between an X-square and a C-square?

An X-square sits diagonally next to a corner: b2, g2, b7 or g7. A C-square sits along the edge next to a corner, like b1 or a2. Both tend to give the corner away, and the X-square is the worse of the two.

What does it mean to have fewer discs but a better position?

It usually means you have fewer frontier discs, the discs that touch empty squares. Fewer frontier discs means fewer places your opponent can play, which means they run out of good moves first. Disc count only matters on the last move.

Are Reversi and Othello terms the same?

Yes. Almost all of the vocabulary on this page came out of competitive Othello since 1971, but it applies to Reversi identically, because the two games share a board and a flipping rule. The differences .

Is a disc called a piece, a counter or a stone?

Disc is the word the rulebook uses and the one players use. Stone belongs to Go. Piece and counter are understood but mark you out as new.

Othello and Reversi Glossary: 39 Terms Explained

Reversi and Othello share every one of these words. Most of them were coined by competitive Othello players after 1971, but they describe things that were always true of the game.

Access: Whether you can actually reach a square you want. A corner is no use to you if every square that would let you play it is filled with your own discs. Good players talk about keeping access to a region rather than about the region itself.
Anti-Othello: The same game with the result reversed: the player with fewer discs at the end wins. The rules of movement do not change at all. It is harder than it sounds, because giving discs away deliberately is not the same as playing badly. Anti-Othello in full.
Board: An eight by eight grid, 64 squares, traditionally green. Squares are named a1 to h8 with letters left to right and numbers top to bottom. Setting one up.
Book: The body of published opening sequences. A game is "in book" while it still matches a known line and you have "left book" the moment it does not. Leaving book early is normal and is not a mistake. The openings index.
C-square: One of the two squares along an edge next to a corner: b1 and a2 beside a1, and so on for each corner. Playing one while the corner is empty often lets your opponent take the corner. Less dangerous than an X-square, but the same idea. Both explained.

The eight C-squares, two beside each corner.
Corner: a1, h1, a8 or h8. A disc on a corner can never be flipped, because no line runs through a corner with squares on both sides of it. Corners are the single most valuable squares on the board and most games turn on them. Corner strategy.

The four corners. Nothing placed here can ever be flipped.
Diagonal opening: The family of openings that begin 1. f5 f6. White's reply puts three white discs on a diagonal line across Black's row. Diagonal openings.

After 1. f5 f6. White holds d4, e5 and f6 on a diagonal.
Disc: One playing piece, black on one face and white on the other. There are 64 in a set and each player takes 32. Discs are never removed from the board once played; they only flip. Not "counter", not "chip", and not "stone", which belongs to Go.
Draw: A finished game with 32 discs each. Draws are uncommon but perfectly normal, and the 2023 computer proof says a game with no mistakes on either side ends in one. Scoring.
Edge: Any square on row 1, row 8, the a file or the h file. Edge discs can only be flipped along the edge itself, which makes them safer than middle discs but not safe. An unfinished edge is one of the most common ways to lose a corner. Edge play.
Endgame: Roughly the last fifteen moves, once the shape of the board is settled and the game becomes a counting exercise. Strong programs solve the endgame exactly rather than estimating it. Endgame strategy.
Evaluation: A number a computer gives a position to say who is better and by how much. Evaluations weigh corners, mobility and stable discs far more heavily than disc count. How the computer thinks.
Flip: Turning a disc over so it shows the other colour. Every disc trapped between the disc you just played and another of your own, in an unbroken straight line, flips. A move is only legal if it flips at least one disc. Legal moves.
Forced move: A turn where you have exactly one legal square. Forcing your opponent into a position where their only move is one they hate is a large part of good play, and it is the practical use of mobility.
Frontier disc: A disc that touches at least one empty square. Frontier discs give your opponent somewhere to play, so a large frontier is a weakness. Keeping your frontier small is what "fewer discs is better" really means. Mobility.
Interior disc: A disc with no empty square next to it in any of the eight directions. Interior discs cannot be used against you as a landing point, so they are the discs you want. The opposite of a frontier disc.
Legal move: An empty square where placing your disc traps at least one line of opposing discs between it and another disc of yours. If a square flips nothing, you cannot play there, no matter how much you would like to. Worked examples.
Midgame: The stretch between the opening and the endgame, roughly moves fifteen to forty-five. This is where mobility and frontier control decide the game, and where most games are actually won and lost. Strategy guide.
Mobility: How many legal moves you have right now. More is better, because it means you can pick a good square instead of playing the only one left. Reducing your opponent's mobility is the main plan in most strong games. Mobility explained.

Black to move with fourteen legal squares, all marked, and fewer discs than White.
Notation: The naming system for squares: a letter for the column, a to h left to right, and a number for the row, 1 to 8 top to bottom. a1 is the top left corner. Notation in full.
Opening: The first ten to fifteen moves, and also the name given to a specific sequence of them. There are 71 named openings in the standard catalogue, sorted into three families. All of them.
Outflank: The official word for trapping a line of opposing discs between the disc you are placing and one you already own. The rulebook says a legal move must "outflank and flip" at least one disc. Official rules.
Parallel opening: The family of openings that begin 1. f5 f4, where White's three discs end up in a row alongside Black's. Strong players avoid it and it has no catalogued continuations. Parallel openings.

After 1. f5 f4. Two flat rows facing each other.
Parity: Who gets to play the last disc in a region of empty squares, and often the last disc of the whole game. Moving second into an empty pocket is usually worth several discs at the finish. Parity explained.
Pass: What happens when you have no legal move: your turn is skipped and your opponent plays again. You are never allowed to pass by choice, and you rejoin as soon as a legal move appears. The passing rule.
Perfect play: Play that never lets the result get worse than it has to be. Othello was weakly solved in 2023 and perfect play from the start gives a draw, so neither colour has a proven advantage. Is Othello solved?
Perpendicular opening: The family of openings that begin 1. f5 d6, where White's three discs form a vertical line at right angles to Black's row. The largest family and the best documented. Perpendicular openings.

After 1. f5 d6. White holds d4, d5 and d6 in a column.
Ply: One move by one player. A game of Othello runs to sixty plies if the board fills. Computer people count search depth in plies, so "eight ply" means looking four moves ahead for each side.
Potential mobility: How many moves you are likely to have soon, rather than right now. It is usually measured by counting the empty squares next to your opponent's discs, since those are the squares that may become playable for you. More on mobility.
Quiet move: A move that flips as few discs as possible, usually one, and leaves your position compact. Quiet moves keep your frontier small and your opponent short of options. They are the backbone of good opening and midgame play. Opening strategy.
Stable disc: A disc that cannot be flipped for the rest of the game, no matter what happens. Corners are always stable, and discs anchored in an unbroken run back to a corner usually are too. Counting stable discs is far more useful than counting all discs. Stable discs.
Sweep: An informal term, used loosely and not standardised. Where it appears it usually describes a single move that flips a whole line at once, most often a complete edge. You can play for years without needing the word.
Tempo: Who is being made to move. You have the tempo when your opponent is spending turns answering you rather than doing what they want. In Othello it is closely tied to parity, because being forced to move first into an empty region usually costs you.
Transcript: A written record of a game: the squares in the order they were played, usually run together with no separators, like f5d6c3d3c4. Passes are left out. The board on this site reads and writes exactly this. Notation.
Unbalanced edge: A run of your discs along an edge that is off centre, with both corners still empty. The overhang gives your opponent a place to wedge in, and the wedge usually costs you a corner. A balanced run is much harder to attack. Edge play.
Unstable disc: Any disc that can still be flipped. Most discs are unstable for most of the game, which is why the score at move thirty tells you almost nothing.
Wedge: Playing into a gap on an edge so your disc sits between two of your opponent's. A wedge cannot be flipped along the edge, and it usually breaks the edge open and wins a corner. It is the standard punishment for an unbalanced edge. Edge play.
Wipeout: A game that ends because one player has no discs left on the board at all. Play stops immediately. In tournaments a wipeout scores the maximum, 64 to 0. The shortest possible game is a nine-move wipeout. Scoring.

The shortest game possible: nine moves, and White has no discs left.
X-square: b2, g2, b7 or g7, the square diagonally next to a corner. Playing one while that corner is empty usually hands the corner over within a few moves. The most expensive square on the board for a beginner. Why they hurt.

The four X-squares, one diagonally inside each corner.

Where to go next

The rules, if any of the basics above were new.
Strategy guide, the six important terms put to work.
Notation, how squares and games are written.
Openings, all 71 named sequences.
Reversi vs Othello, why the game has two names.

Othello and Reversi glossary

Where to go next

Frequently asked questions