Dominosa
Reconstruct the hidden set of dominoes
Click toward the nearest edge of a number to toggle a domino; right-click near an edge to toggle an exclusion line. Arrow keys move by half a cell. Enter places/removes a domino; Space toggles a line. Digit keys highlight up to two values; Escape clears highlights. Ctrl/⌘+Z undoes; Ctrl/⌘+Shift+Z or Ctrl/⌘+Y redoes. Click or tap the centre of a number then use direction controls, or tap near an edge directly.
Progress saves on this device.
Highlight up to two numbers. Matching cells carry a 1 or 2 marker. Right-click a number to toggle its highlight.
How to play
Cover the rectangle with dominoes, each spanning two orthogonally adjacent numbers. Use each unordered pair exactly once, including doubles and zeroes. Duplicate pairs carry ! marks. Every number must be paired.
Placing an overlapping domino removes the old dominoes and nearby exclusion lines. Lines record pairs you have ruled out and are available only between unpaired numbers. Tap the same edge again to remove a domino or line.
Solve reveals the upstream solution and does not count as a win. Undo restores previous work. Restart clears moves and history on this puzzle. Highlighted numbers are a temporary viewing aid.
Puzzle credited to O. S. Adler. Engine: Simon Tatham’s Dominosa. MIT license and credits.