Reversi

Othello rules · 8 × 8

Black to move

2Black

2White

Opponent

Show legal moves Coach · grades your moves and explains why

Rules

The game starts with four discs in the centre: white on d4 and e5, black on d5 and e4. Black moves first.
Place a disc of your colour so that one or more straight lines (horizontal, vertical or diagonal) of your opponent's discs lie between it and another of your discs. Those discs are outflanked and flip to your colour.
Every move must flip at least one disc. Discs are flipped in all directions at once, but only along the lines closed by the new disc.
If you have no legal move, you pass and your opponent plays again. You may not pass if a move is available.
The game ends when neither player can move — usually when the board is full. The player with more discs wins.

How the computer plays

Opening book. Knows 76 named openings (Tiger, Rose, Buffalo, Cow…) and varies between them, but only plays a book move its own search agrees with.
Mobility. Prefers positions where it has more moves than you and you have fewer.
Frontier and quiet moves. Avoids discs next to empty squares and favours moves that flip only interior discs, so you get fewer openings.
Evaporation. Early on, it keeps its disc count low. Discs only start to count near the end.
Corners, X- and C-squares. Corners score highest. It avoids X-squares and C-squares next to an empty corner.
Edges. Scores every edge pattern: rewards balanced edges and anchored runs, and penalises unbalanced edges and single holes you could wedge into (Stoner-trap targets).
Stable discs. Counts discs that can never be flipped again.
Parity. Late in the game it tries to get the last move, playing into odd regions first.
Perfect endgame. On Hard it solves the last 16 empty squares exactly (10 on Medium). Easy plays greedily with random slips.