Play Dominosa Online Free

Dominosa is a domino fitting puzzle disguised as a rectangular grid of numbers. You must divide adjacent cells into dominoes so that every unordered pair from the set appears exactly once. In a double-six puzzle, for example, 2-5 and 5-2 name the same piece, and each pair from 0-0 through 6-6 occurs once. The task develops exact-cover reasoning and candidate elimination because choosing one boundary removes a piece and two cells from every rival placement. Find a rare pair and start fitting the set online.

1. Identify the set's highest number, n. The complete set contains every unordered pair a-b where 0 is at most a, a is at most b, and b is at most n. Each pair must appear once.
2. Inspect horizontally and vertically adjacent cells as candidate dominoes. Record the pair without order, so a neighboring 3 and 5 represents the same 3-5 piece whichever cell comes first.
3. Place a domino when its pair occurs in only one available adjacency. Cover those two cells, mark the pair used, and remove every candidate that overlaps either covered cell.
4. Also inspect cells with only one remaining neighbor and pairs whose alternative placements overlap forced pieces. Continue alternating cell constraints with pair constraints rather than scanning only repeated number combinations.
5. Finish when dominoes cover every cell exactly once and each unordered pair in the complete set appears exactly once. Review the pair inventory to catch a duplicated orientation that looks different on the grid.

• Every domino covers exactly two cells that share a horizontal or vertical edge. Diagonal cells cannot form a piece.
• Every grid cell belongs to exactly one domino. Pieces cannot overlap, and no cell remains uncovered in the final tiling.
• Each unordered number pair appears exactly once. The pairs 1-4 and 4-1 are identical for this rule and cannot both be used.
• Doubles such as 3-3 also appear once. Their two cells still need an ordinary shared edge and obey the same coverage restrictions.
• For a set numbered 0 through n, the board represents (n+1)(n+2)/2 dominoes and therefore has (n+1)(n+2) cells. The printed arrangement hides their boundaries.

• Build a pair-frequency table. If an unused pair occurs in only one still-available adjacency, place it immediately. Recount after every placement because overlap removal may make another pair unique.
• Track cell degree. A cell with one available neighbor forces that domino even when its pair appears elsewhere. The placement then consumes the pair, eliminating those other occurrences.
• Use doubles as anchors. Equal numbers often have fewer adjacent matches than mixed pairs. Check each double early, but confirm that its placement does not steal a cell needed by an even rarer pair.
• Look for mutual exclusion. If every candidate for pair A overlaps one particular candidate for pair B, test the consequences of choosing B. Candidate graphs expose chains that a visual scan can miss.
• Apply region parity. Any area cut off by fixed domino boundaries must contain an even number of uncovered cells. Reject a tentative placement that isolates an odd pocket, regardless of whether its number pair remains legal.

• Shikaku: Shikaku partitions every cell into rectangles of clue-given areas. Dominosa uses only two-cell rectangles but also requires one copy of every number pair. Both reward exact coverage and stranded-cell checks.
• Kakurasu: Kakurasu selects weighted cells to satisfy sums, while Dominosa selects adjacent pairs to satisfy a complete inventory. Both become clearer when players maintain candidates and propagate each forced choice.

• Two independent constraints drive each solve. Every cell needs a partner, and every pair needs one location. Progress resumes whenever one side of that relationship becomes unique.
• The visible numbers make checking direct. Players can list the complete domino set and verify every placed piece without relying on hidden clues or vocabulary.
• Small placements create broad effects. One domino may remove several overlaps and consume a frequently occurring pair, turning a crowded grid into a chain of forced fits.

Play Dominosa online for free with a numbered grid and a complete pair inventory beside it. Select adjacent cells to place a domino, remove a tentative boundary when needed, and watch used pairs update. The browser reduces bookkeeping while leaving the exact-cover decisions to you. Start with an easy set to learn unordered pairs, then choose a larger board where candidate frequency and region parity guide the solve.

Dominosa asks you to recover one complete domino set from a number grid. Treat every pair as unordered, cover each cell once, and use each piece once. Scan rare pairs, low-degree cells, doubles, and odd pockets before trying a guess. Simon Tatham's documentation gives a clear rules reference for the format. Open a puzzle above, place the pair with one legal location, and let each removed overlap reveal the hidden tiling.

• Dominosa documentation (Simon Tatham's Portable Puzzle Collection)