Reversi · Othello (2 players) is the classic disc-flipping game for two people sharing one phone, tablet or computer. Black and White take turns on an 8×8 green board; whenever you trap a line of your opponent's discs between two of your own, the whole line flips to your colour, and the player with more discs when the board is full wins. Every legal move is marked with a dot and the turn passes automatically when a player is stuck, so a complete beginner can finish a proper game on the first try. No account, no download — it opens straight in the browser.

How to play

Skim the four-line rule summary on the title screen and tap Play together. The board opens in the standard position — two black and two white discs in the centre, arranged diagonally — and Black moves first.
Small dots in the current player's colour show every square that can be played. Tap one and your disc lands there; the trapped discs then flip one by one, starting with the ones closest to your move. The square you just played is highlighted in yellow so both of you can see the last move at a glance.
Tap a square without a dot and a red outline flashes with the message "Not a legal move — you must flank at least one disc". The board doesn't change; just choose another square.
At the top, the Black and White player cards show a live disc count for each side, and the card of whoever is to move glows with a lime border. Between the cards sits the move counter, with a one-line status message underneath.
If a player has no legal move, the app shows "Black passed" (or "White passed") and hands the turn straight back to the other side — there's no button to press. When the board is full, or neither player can move, the game ends.
The result overlay shows the winner with a trophy (or a handshake for a tie), the final count such as "Black 34 : White 30", and Series today — a running tally of wins, losses and draws for as long as this page has been open. Choose Play again, Share result to send a one-line summary and link through your device's share sheet (or copy the link), or Title to go back to the start.

Rules

Reversi has essentially one rule: a disc placed so that a straight line of opponent discs is trapped between it and another of your discs flips that entire line. The line can run horizontally, vertically or diagonally, and one move can flip lines in several directions at once. Everything else follows from that.

Board and start — 8×8 squares, 64 in total. Two discs of each colour sit in the centre in a diagonal pattern, and Black always begins.
Legal moves — you may only play on a square that flanks at least one opponent disc. The app shows every such square with a dot, so you never have to work it out yourself.
Flipping — every trapped disc flips, not just the ones you'd like. Flipped discs stay that colour until they are trapped again.
Passing — if you have no legal move, your turn is skipped and your opponent plays again. You cannot pass voluntarily while a legal move exists.
Game end — the game is over when all 64 squares are filled or when neither side can move. More discs wins; an equal count is a draw.

Undo	Takes back the last move, including every disc it flipped. There is no limit on how many times you can undo.
New game	Resets to the starting position. If a game is in progress you'll be asked to confirm first.
Resign	Tap once and the button changes to "Really resign?"; tap again within 2.5 seconds and the player to move concedes the game.

Tips for winning

New players tend to grab the move that flips the most discs. Reversi punishes that: only the final count matters, and a big lead in the middle of the game usually means you've run out of good moves later. A few ideas that actually help:

Corners are gold. A corner disc can never be flanked, so it stays yours for the rest of the game and anchors the edges next to it. Avoid playing on the squares that touch a corner — especially the diagonal one — until the corner is settled, because that usually hands the corner to your opponent.
Count the dots, not the discs. Having more legal moves than your opponent — what players call mobility — is the real advantage. If your opponent is down to one or two dots, they're being forced into bad squares or a pass.
Stay compact and central early on. Fewer discs, clustered in the middle, leave you flexible. Discs spread to the outside open up new squares for the other side.
Treat edges with respect. Edge discs are hard to flip, but an edge with a gap in it can be swept in one move. Fill an edge when you can hold it, not just because it's available.
Use Undo to explore. Wondering what a move does? Play it, watch the flips, and take it back. Unlimited undo makes this a great way for two people to learn together.

FAQ

What happens when I have no legal move?

The app skips your turn automatically, shows a "passed" message, and your opponent moves again. If neither player can move, the game ends right there and the discs are counted.

I placed a disc but nothing flipped.

In Reversi a disc only flips when it's trapped between two of the other colour. This app only lets you play on squares that trap something, so a move always flips at least one disc. If it isn't obvious which ones, look from the yellow square outward in all eight directions for a line ending in your own colour.

Why does everyone talk about corners?

A corner has the board edge on two sides, so there's no way to place discs on either side of it. Once you own a corner it can't be flipped back, and the discs you build out from it along the edges become safe too. That's why experienced players give up plenty of discs to secure one.

How many times can I undo?

As many as you like. Each tap of Undo takes back one move and restores every disc that move flipped. It's handy when playing with kids or when you're both still learning.

Can I play online or against the computer?

No — this is a two-player game on one device. Put the phone on the table between you, or pass it back and forth, and take turns.

Is Othello different from Reversi?

They're the same game in all the ways that matter. Reversi is the older name, from 19th-century England; Othello is a trademarked name from 1970s Japan that fixed the diagonal starting position everyone uses today. This app follows that modern standard.

Good alongside this

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Chess (2 players) — full rules including castling, en passant and promotion, with automatic checkmate detection.
Janggi · Korean Chess (2 players) — Korean chess with a face-to-face mode for players sitting opposite each other.
Alkkagi · Stone Flicking (2 players) — pull back and release to flick go stones off the board.
Minesweeper — a solo classic for when your opponent has gone home.