Dominosa Logic Puzzle

Dominosa is a logic puzzle about finding a complete set of dominoes in a number grid. You can play grids from 5×4 up to 11×10, and every puzzle here has exactly one tiling that you can reach by logic, with no guessing. People also look this up as a dominosa puzzle, a domino puzzle, or a printable dominosa puzzle.

Rules

The board is a tall rectangle. 5×4 uses the numbers 0 through 3. Larger grids use a higher top number. Each number appears the same number of times. A domino covers two cells that share a side. Each pair of values appears exactly once, including two copies of the same number. Every cell belongs to one domino.

A wall marks a boundary you have ruled out. Walls are optional. An empty edge is simply not a domino. The numbers are given. You do not change them.

The puzzle is solved when the domino edges match the one solution. A repeated pair, a cell sitting in two dominoes, or a number with no legal neighbour left is a mistake.

Controls

Choose Domino or Wall under the board. Tap the boundary between two cells to set that edge to the mark you picked. Tap an edge that already has that mark to clear it. You can also tap one cell, then a cell that shares a side, which is the easier path on a small screen. A drag from one cell onto a neighbour does the same. Cells after that first step are ignored, so one stroke places one edge.

With a mouse, right-click an edge to place a wall, including when Domino is selected. A second right-click leaves the wall in place. Tap that wall while Wall is selected to clear it. Right-click on a number does not place a wall.

On a keyboard, the arrow keys move the cell cursor. Hold Shift and press an arrow to apply the active mark to the edge in that direction. M switches Domino and Wall. The size menu lists 5×4 through 11×10, and the difficulty menu lists easy, medium, and hard.

A domino or a cell turns red when a pair is repeated, a cell is in two dominoes, a number has nowhere to go, or a finished tiling is not the solution. Press Hint for one step the rules already force, with a sentence saying why. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the marks you placed. The numbers stay. When the dominoes match, the board turns teal. The timer and your progress stay on this device.

Techniques

Easy grids follow from one forced placement:

• Unique pair: this pair fits in only one place.
• Only partner: this cell has only one neighbour it can join.
• Same pair: every way to cover this cell is the same pair, so that pair is removed everywhere else.
• Must overlap: every remaining place for this pair covers this cell, so the cell cannot take another pair.
• Parity: this placement would leave an odd region, so it is removed.
• Confine: these cells of one number only have room for this set of pairs.
• Exclude: the other side contradicts the board, so this domino is placed.
• Agree: both sides force this same domino.

Beginner tips

Start with a pair of numbers that can sit together in only one place, and lay that domino. Then look for a cell whose open neighbours have all been ruled out except one. When two possible dominoes for a cell use the same pair, cross that pair off everywhere else. Walls are notes. You can solve with dominoes alone.