How to play Duo

Duo is a 6×6 grid where every cell ends up holding a sun or a moon. You are never asked to guess: every board has exactly one solution, and it can always be reached by reasoning from what is already on the grid.

The five rules

One deduction, step by step

Those five rules are enough to fill a whole board. Here is the reasoning that does most of the work, on a single row.

Where to start

Look for pairs before anything else. Two identical symbols next to each other, or two of them with a gap between, decide a third cell on their own — those are the cheapest moves on the board.

Then count. Any row or column with three of one symbol is finished for that symbol: fill the rest with the other one. On a 6×6 grid this cascades quickly, because each cell you write belongs to a row and a column at once.

On easy boards the signs do a lot of the work, so read them before you look at the symbols. On hard boards there are fewer of them and the deduction goes deeper, but it is still there: you never need to try a value and see what happens.

Questions people ask

Is Duo the same as Takuzu?

Yes. The puzzle is played as Takuzu, Binairo and Binary Puzzle, usually with ones and zeros. Puzlia uses a sun and a moon because two shapes are easier to tell apart at a glance than two digits, and adds = and × signs between cells.

Can a Duo puzzle have two solutions?

Not here. Every board is generated and then checked to have exactly one solution before it is served, at all three difficulties.

What changes between Easy, Medium and Hard?

How much help you start with: Easy comes with ten signs and ten given cells, Medium with fewer of both, and Hard may give you no cells at all and asks for deeper reasoning. The rules never change.

Do I need to guess on Hard?

No. Hard boards need longer chains of reasoning — sometimes ruling out a value by following its consequences — but a solvable path is always there.

Today's Duo Start on Easy

The other three