Dominosa

What is Dominosa?

Dominosa is a logic puzzle made from a full set of dominoes. Every domino of the set lies face up on the board, packed edge to edge, but the lines between the dominoes have been rubbed out: all that is left is a grid of pips. Your job is to find the joins again, pairing the cells two by two so that every cell is covered and every domino of the set appears exactly once. The puzzle goes back to the nineteenth century and is often simply called a domino puzzle.

Every board has exactly one answer, and it can always be reached by reasoning, never by guessing. There are three sets: double-three on 4 by 5 cells (10 dominoes), double-five on 6 by 7 (21 dominoes) and double-six on 7 by 8 (28 dominoes). A new board, the same for everyone, arrives every day.

How to play

Tap a cell, then a cell next to it, and a domino is laid on the two. You can also drag from a cell toward its neighbour and let go.

• Laying. While you drag, the domino you are forming is outlined and named under the board. A domino laid over others takes their place.
• Taking away. Tap a domino on the board to take it away.
• The set. The full set sits under the board, or beside it with your phone held sideways. Every domino you lay is greyed out there, so you can see at a glance which ones are still missing.
• Red dominoes. Every domino of the set appears only once. Lay the same one twice and both copies turn red, on the board and in the set.
• Undo. Undo takes back your last change, and puts back any dominoes it replaced.
• Score. Your score is the time to lay the whole set, plus 30 seconds for every hint. Every set has its own leaderboard.

Strategy tips

1. Hunt for the rare pairs. A domino whose two numbers sit side by side in only one spot on the whole board has to go there. Doubles are a good place to start looking.
2. Mind the corners. A cell in a corner has only two neighbours, and once one of them is taken it has one. A cell with one partner left takes that domino.
3. Strike out what is used. Once a domino is laid, every other spot where the same two numbers meet is out of play. That often leaves a nearby cell with only one choice.
4. Shared cells. When every spot left for a domino covers the same cell, that cell belongs to it, and no other domino can use it.
5. Count the cells. Dominoes always cover two cells, so a closed patch of free cells must have an even count. A domino that would wall off an odd patch is wrong. On the double-six boards, follow a choice a few dominoes ahead to see whether it runs into a dead end.

FAQ

Is Dominosa free to play?

Yes. Like every game on Play, it runs free in your browser, with no download and no account needed.

Can the same domino appear twice?

No. A board holds every domino of its set exactly once. If you lay a domino twice, both copies turn red until you move one of them.

What does a hint do?

It takes away a domino that is not part of the answer first, if there is one; otherwise it lays the next domino that follows by logic from the board, and the line under the board says why. Each hint adds 30 seconds to your time.

What is today's board?

Every day everyone plays the same double-five board of 6 by 7 cells, so times compare directly on the leaderboard.

How is my score counted?

Your score is the time from the first look at the board to the last domino that completes it, plus 30 seconds for every hint. Lower is better.