What is dominosa?
Dominosa is a logic puzzle made from a set of dominoes. Someone laid out a complete set in a rectangle, wrote down the numbers, and took the pieces away. All you see is a grid of numbers. Your job is to work out where each domino was, using the fact that every pair of numbers appears exactly once in the set.
It is quiet, satisfying detective work. Play dominosa online free here, from tiny warm-up grids to large boards with numbers up to 9.
Where dominosa comes from
The puzzle is widely credited to O. S. Adler, and the name dominosa takes part of its letters from those initials. It is built on the ordinary domino set: the standard double-six set has 28 pieces, one for every pair from 0-0 to 6-6, and fills an 8×7 rectangle exactly. Bigger sets with higher numbers make bigger grids.
How to play here
Pick a size and level on the start card, then press Play. Click or tap between two neighboring numbers to place a domino over them, and click again to remove it. If you place a domino over one that is already there, the old one is removed.
To note that two numbers are not one domino, right-click between them with a mouse, or turn on Mark in the dock and tap between them on a phone. That draws a line you can use as a reminder. Right-clicking a number, or tapping it with Mark on, highlights every copy of that number on the grid, which helps when you hunt for a pair.
Undo, Redo and New puzzle are in the dock. The pause menu has Restart, Solve and Enter game ID.
Counting pairs: the key idea
The whole puzzle rests on one fact: each pair appears once. On a double-six board there are 28 pairs, and the grid offers far more places where each pair could go. Your job is to cut those options down until only one remains for each.
A good habit is to pick a pair, say 4-1, and find every spot where a 4 sits next to a 1. If there is only one, place it. If there are two, check whether one of them would leave a neighboring square with nowhere to go.
A step-by-step way to solve
- Place the unique pairs. Any pair that can only be made in one spot is fixed.
- Fill the trapped squares. A square with only one free neighbor must pair with it.
- Block repeated pairs. After placing a domino, draw lines across every other spot with the same two numbers.
- Watch for doubles. A double like 3-3 can only go where two 3s touch, and there are often very few of those.
Why play dominosa?
Dominosa uses objects everyone knows, but it plays nothing like a game of dominoes. It is pure deduction with a clear finish. If you like it, try Magnets, another domino-grid puzzle, or Hitori.
How to play Dominosa
- A full set of dominoes, one of every pair of numbers from 0 up to the highest number, was laid out to fill the rectangle. Then the dominoes were taken away and only the numbers were left.
- Your job is to put them back by covering the grid with dominoes, each one covering two numbers side by side.
- Every pair appears exactly once. You can't use 2-5 twice, and 5-2 counts as the same domino.
- Dominoes can't overlap, and every square must be covered.
Dominosa tips
Find pairs that appear only once
If a pair like 3-6 can only be made in one spot on the whole grid, that domino must go there. Scan for these first.
Look for squares with one way out
A square in a corner, or one surrounded by placed dominoes, may have only one free neighbor left. It has to pair with that one.
Cross out used pairs
Once a domino is placed, every other spot where the same pair could go is ruled out. Draw a line between those squares so you don't try them again.
Frequently asked questions
How many dominoes are in a double-six set?
Twenty-eight, from 0-0 up to 6-6. They cover 56 squares, which is why the standard dominosa board is 8×7.
Who invented dominosa?
The puzzle is widely credited to O. S. Adler, and part of its name comes from those initials.
Does a dominosa puzzle have only one solution?
Yes. The boards here are generated with exactly one answer, so you can always solve them by logic.
