Sun & Moon

How to play Sun & Moon

Sun & Moon is a binary logic puzzle. Every square on the board ends up holding either a sun or a moon, and three simple rules decide which. The puzzle gives you a few squares to start with and a handful of signs between squares, and the rest is deduction.

The same kind of puzzle is known as Takuzu, Binairo or simply a binary puzzle. Every one here has exactly one answer, and every one can be finished from the rules alone, with no guessing.

The rules

• Balance. Every row and every column holds as many suns as moons — three of each on a 6 x 6 board.
• No three in a row. No more than two of the same may sit next to each other, across or down.
• Signs. An = between two squares means they are the same. An × means they are different.

Controls

Tap an empty square once for a sun, again for a moon, and a third time to clear it. The squares the puzzle started with are shaded and cannot be changed.

Mistakes show up as you make them: three alike in a row, too many of one kind in a line, or a sign that is not being kept all turn red. The board does not tell you when a square simply disagrees with the answer — only when a rule is actually broken — so the reasoning stays yours.

Undo takes back your last move, Clear returns the board to how it started, and Hint puts one square right — a wrong one if there is one, otherwise an empty one.

How to solve it

Those four ideas are enough for every puzzle here. The puzzles are built so that they are.

• Two alike make a wall. Two suns side by side must have moons at both ends, and the same the other way round.
• Fill the gap. A sun, a gap and another sun means the gap is a moon, or there would be three suns together.
• Finish a full line. Once a row has all three of its suns, every other square in it is a moon.
• Follow the signs. A sign next to a square you know tells you its neighbour straight away. Chains of signs can carry one known square right across the board.

Sizes and the daily

Free play offers the classic 6 x 6 and a larger 8 x 8. The daily puzzle is always 6 x 6, so that times from one day to the next can be compared. It is the same for every player, changes at midnight in your own time zone, and keeps a streak.

How the puzzles are made

A full answer grid is built first, keeping the balance and no-three-in-a-row rules. The puzzle then starts with every square filled and a sign on every edge between squares, and clues are taken away one by one in random order — squares and signs mixed together.

After each removal the game tries to finish the puzzle using only the rules above, with no trial and error. If it can, the clue stays gone; if it cannot, the clue goes back. What is left is a puzzle with only a handful of clues that can still be solved by reasoning alone — which also means it has exactly one answer.

Common questions

Do I ever need to guess?

No. Every puzzle is checked to be solvable from the three rules alone before you see it.

Do rows have to be different from each other?

No. Some versions of this puzzle ban two identical rows, but here the only rules are balance, no three in a row, and the signs.

Why did a square turn red?

Red means a rule is broken: three alike together, too many of one kind in a row or column, or a sign that does not hold. Fix the red squares and the rest of the board is still fine.

Is there a new puzzle every day?

Yes, at midnight in your own time zone, the same for everyone. Free play gives you as many more as you like.