Strategy
Mobility in Reversi
The short answer
Mobility is the number of legal moves available to you. Keeping more legal moves than your opponent is the main plan of the middle game, because a player with almost no choices is eventually forced to open a corner. You build mobility by flipping as few discs as possible.
Reversi is decided by who runs out of good moves first. Everything else follows from that one fact.
What mobility is
Count the squares you are legally allowed to play on. That number is your mobility. Compare it with your opponent's and you have the most useful single reading of a position available without a computer.
A move is legal only when it traps at least one enemy disc in a line. So every legal move you have depends on where your opponent's discs are, and every legal move they have depends on where yours are. Adding discs of your colour along the edge of the empty space adds moves for them, not for you.
The mobility trap, on a real board
Below, Black has played twenty greedy moves, always taking the biggest flip available, against the Intermediate computer. Black leads 18 to 6.
Same board, two very different pictures. Black has 5 squares to choose from. White has 15. Black's discs are spread right across the empty space: 13 of them touch an empty square, against 4 for White. Each of those Black discs is a square White is allowed to play beside.
That game ended 0 to 64. Black never recovered, because the position could not be fixed by playing better from there. The damage was structural.
Quiet moves
A quiet move flips one or two discs, sits inside your own area, and barely changes the outline of your position. It is the tool that builds mobility.
In the position above, Black's remaining moves flip b1 (1), b2 (3), b5 (2), b6 (3), d6 (1). The small ones are the good ones. A move that flips one disc adds one square to your frontier at most; a move that flips six can add half a dozen.
The habit to build is simple and it takes about ten games to stop feeling wrong. When two moves look about equal, take the one that flips fewer discs.
Frontier discs are the mechanism
A frontier disc is one that touches at least one empty square. An interior disc is completely surrounded, so nothing can be placed next to it and nothing can flip it in from outside.
Your frontier is the surface your opponent plays against. Reduce it and you reduce their mobility. This is why "flip fewer discs" and "keep more moves" are the same instruction: a big flip converts a row of your opponent's interior discs into your own frontier discs.
Squeezing, and how it ends
The plan is to keep the gap open until your opponent has nothing safe left. Below is a normal game, 49 moves in and almost level on discs at 26 to 27. White has 4 legal moves and Black has 5, and no corner has been taken.
Nine moves later Black is down to 1 legal move, and it is the X-square b2, which opens the a1 corner.
Black is not blundering here. Black has no choice. That is the point of the whole plan. X-squares and C-squares covers what happens next, and parity covers who is forced to move last once the board gets this tight.
How to count it in a real game
- Count your legal moves. The board on this site marks them, so it is just counting dots.
- Pick two or three candidate moves. For each one, work out roughly how many squares it opens up for your opponent.
- Prefer the one that opens the fewest. When they are equal, prefer the one that flips fewer discs.
- Never fix a mobility problem with an X-square. Running out of moves is bad; giving away a corner is worse.
Try it against the computer
Play the computer on Intermediate. Before each move, count the dots. After the computer replies, count them again. If your number keeps dropping while theirs stays high, you are being squeezed, and the answer is smaller moves rather than bigger ones.
Next: parity, endgame strategy, corner strategy, or back to the strategy guide.
Frequently asked questions
What is mobility in Othello?
Mobility is the number of legal moves you have. Having more legal moves than your opponent means you choose the shape of the game and they react to it. It is the single most useful thing to count in the middle of a game.
Why does flipping fewer discs give me more mobility?
Because a legal move has to sit next to an opponent disc. The fewer of your discs sit next to empty squares, the fewer places your opponent is allowed to play. Big flips push your colour outward and hand them a wider choice.
What is a quiet move?
A move that flips one or two discs, usually deep inside your own area, and adds almost nothing to your frontier. Quiet moves keep your options open and take your opponent's away, which is exactly what you want in the middle of the game.
What is potential mobility?
The number of empty squares touching your opponent's discs, which is roughly the number of places you might get to play soon. A player with low current mobility but high potential mobility is often about to recover.
What happens when I run out of legal moves?
You pass and your opponent moves again. You rejoin as soon as a legal move appears. It is not a penalty in the rules, but it usually means the position has already gone wrong. The passing rule.