Corners, mobility and parity

New reversi players try to flip as many discs as possible. Experienced players are happy to be behind on discs for most of the game. These three ideas explain why.

Corners and stable discs

A disc on a corner can never be flipped: no line can pass through it to trap it. From a corner, whole edges can become stable, and stable discs are the only ones you can really count on.

Diagonal neighbours of an empty corner are traps. A disc on one of them very often lets the opponent into the corner. The edge squares beside a corner carry a smaller risk. Once the corner is occupied, these squares are ordinary again.

Mobility: count moves, not discs

Your position is good when you have many sensible moves and your opponent has few. Every disc you flip in the middle of the board can give the opponent a new move. Quiet moves that flip a single inner disc keep the opponent's choices small.

Eventually a player with no good moves left has to play next to a corner or give up an edge, and that is where games turn.

Frontier discs

A disc next to an empty square is a frontier disc: it gives the opponent something to play against. Walls of your discs facing open space are a liability. Try to make moves that flip discs surrounded by other discs.

Parity in the ending

Towards the end the empty squares split into small regions. Making the last move in a region is usually an advantage, because nothing can flip your disc afterwards. In a region with an odd number of empties, the player who moves into it first tends to get the last move there.

The medium and hard reversi puzzles are exact endings with eight or eleven empty squares. Exactly one move wins in each, and parity is often the reason.