Dominosa
Can I use the same domino pair twice in Dominosa?
Each unordered number pair belongs to the domino set once. Covering the board without overlaps is necessary but not sufficient: repeated pairs leave another required domino missing.
Worked example
The pair 2–5 is the same domino as 5–2. Do not count the reversed order as a different piece.
What to check on your board
Apply the rule above to your current board, then verify the remaining puzzle constraints. The example explains a local deduction; it is not a solution to every randomly generated board.
Checking your next move
Write number pairs in a consistent order, such as smaller number first, while checking the board. This makes reversed duplicates easier to spot. Once a pair is used, look for any other placement that would require the same domino before committing neighbouring boundaries.
Using this reference
For Dominosa, read the rule and the worked example together before opening the game. Separate what the rule explicitly requires from what only seems likely on the current board. A useful next move is one you can explain from the visible constraints, not merely a pattern that looks familiar.
If you get stuck, isolate a smaller part of the position and list what is still possible there. After a move, check the affected rows, regions or connections again. The reference explains the documented rules; progress, controls and save behavior belong to the linked game.
Read the developer’s manual ↗ · Open the developer’s browser version ↗
Manual inspected: · Simon Tatham & contributors · MIT