Reversi is played on an eight-by-eight board with round discs that are dark on one side and light on the other. You win by having more discs of your color than the computer when the game ends. The rules are short, but the best moves are often not the ones that look obvious.

How to play

You play the dark discs and move first. You can choose to play light and go second when you begin a new game. Four discs sit in the middle at the start.

Small dots show every square where you may play. A legal move must trap a line of light discs between the new dark disc and another dark disc. It can be across, up and down, or on a slant. All the light discs trapped in this way flip and turn dark. One move can flip several lines at once.

On a phone or tablet: tap a dot.
On a computer: click a dot.

If you have no legal move, you pass, and the computer plays again. If the computer cannot move, it passes. The game ends when neither side can move, or when the board is full. Then the discs are counted.

The levels

Easy: the computer likes to take as many discs as it can, and about 3 moves in 10 it plays a random legal square. Undo is on.
Medium: it knows that corners and edges are valuable and looks two moves ahead. Undo is on.
Hard: it looks four moves ahead and values corners and its own freedom to move. There is no undo.
Expert: it searches within about 300 milliseconds a move, and plays the last twelve empty squares perfectly. There is no undo.

Today's game

There is one Medium game a day. It is built from the same seed for every player that day. If you leave, it waits for you.

Tips

Corners are precious. A disc in a corner can never be flipped, and it can anchor lines along the edges.
The squares next to a corner are risky early on. If you play one, you may give the computer a way into the corner.
Having fewer discs in the early game is not a weakness. The side with fewer discs usually has more choices, because the other side has to open the lines.
Count your options, not just your discs. A move that leaves the computer with only one legal reply is often a good one.
Edges are useful but can be tricky. Play along them when you can do so without leaving a gap the computer can use.
Near the end, do the sums. Count the empty squares left and look for moves that keep a region for you.
Flipping a lot of discs in the middle of the game often just feeds the computer more places to play.

A bit of history

Reversi dates from the late nineteenth century and was first played in England.

Questions

Why can I not play where I want? A move must flip at least one light disc. Squares with no dot cannot do that.

What if neither of us can move? The game ends and the discs are counted. More discs wins.

Can I be light? Yes. Pick it on the new game card. Light moves second.

What does Undo do? It takes back a move on Easy and Medium. It is switched off on Hard and Expert.

Reversi