Dominosa Solver, Step by Step
Type every number into its square. Dominoes are outlined as they are found.
How to use the dominosa solver
Choose the set your puzzle uses: a double-five set has the numbers 0 to 5 on a grid 7 wide and 6 high, and bigger sets add a row and a column for each number. Type every number into its square. Press Explain the next step: the solver outlines the dominoes that are certain, crosses out pairings that cannot be, and says why. Show the answer lays every domino at once and checks the solution is the only one.
The rules of dominosa
A full set of dominoes, from double blank to double n, has been laid on the grid and the outlines rubbed out, leaving only the numbers. Divide the grid into pairs of neighbouring squares so that every domino of the set appears exactly once. A double-five set has 21 dominoes; a double-six set, the usual box of dominoes, has 28.
How to solve dominosa, step by step
Find the dominoes with one place. Read off where each domino could go; the ones that appear in only one pair of neighbouring squares go there, and that crosses off every other place for the same domino. Find the squares with one partner. A square whose other neighbours are all taken has to pair with the one that is left. Look for shared squares. When every place left for a domino uses the same square, that square belongs to it and cannot pair any other way. When those run out, the solver tries a pairing and keeps it only if it does not lead to a dead end.
How it was checked
The solver was run on all 120 puzzles of the daily dominosa. It found each stored answer and found it to be the only one, and every step it explained was checked against those answers: it never placed or ruled out a domino wrongly. It was also run on larger sets, up to double nine.
More logic puzzles
There is a new daily dominosa every day. The Pips solver places dominoes under number rules, and the shikaku solver divides a grid in the same spirit. For a puzzle you hold in your hands, see our brain teasers. There are more solvers on the puzzle tools page.
Puzzles you can hold
Packing puzzles from the shop: every piece has to fit.
Frequently Asked Questions
How do you solve dominosa?
Look for dominoes that can only go in one place, and squares that have only one neighbour left to pair with. Every domino you place crosses off every other place for the same domino, which usually forces the next one.
What are the rules of dominosa?
A full set of dominoes has been laid on the grid with the outlines removed. Pair up neighbouring squares so that every domino in the set appears exactly once.
How big is a dominosa grid?
For a double-n set the grid is n+2 squares wide and n+1 high: a double-five set, numbers 0 to 5, is 7 by 6 with 21 dominoes, and a double-six set is 8 by 7 with 28.
Does every dominosa have one solution?
A well-made one does. The solver checks, and says if the numbers you typed allow more than one.