Dominosa Puzzle Online
Learn the rules and deduction patterns behind a uniquely verified Dominosa board, then open Easy 5×4, Standard 7×6 or Challenge 8×7 Practice. Every bundled source is project-generated and independently proved to have one exact-cover completion.
What is Dominosa?
Dominosa is a domino-placement logic puzzle built from a rectangular grid of numbers. Your job is to divide the grid into orthogonally adjacent two-cell dominoes. Every cell must belong to exactly one domino, and every unordered numeric pair allowed by the level must be used exactly once. On an Easy board using 0 through 3, the required inventory is 0–0, 0–1, 0–2, 0–3, 1–1, 1–2, 1–3, 2–2, 2–3 and 3–3. That creates ten dominoes and twenty cells. Standard expands to 0 through 5 and Challenge to 0 through 6. The rule is simple to state, but the interaction between cell coverage and pair inventory creates a compact exact-cover puzzle with strong deduction depth.
Start with pair inventory, not guessing
A productive solve begins by treating every numeric pair as a limited resource. Scan the board for a rare pair such as 0–0 or 5–5. If that pair appears along only one currently available edge, the domino is forced. Once you place it, every other candidate edge touching either cell disappears, and every other appearance of the same numeric pair becomes impossible. This cascading inventory pressure is more informative than pairing cells because two values happen to look convenient. The live ledger on the game page shows whether each pair type is already placed and how many candidate edges remain after your current pairs and X marks.
Use cell pressure as a second deduction channel
Pair inventory is only half the puzzle. Every cell must be covered exactly once. If an unpaired cell has only one available neighboring edge, that edge is forced even when its numeric pair still has several appearances elsewhere. Conversely, if a cell loses all candidate neighbors, you have created a contradiction and should undo or inspect nearby X marks. Strong Dominosa solving alternates between these two views: pair-type scarcity and cell-partner scarcity. V0.130 surfaces both kinds of pressure in the same monitor so the hints remain explainable instead of appearing as unexplained answer reveals.
X marks make elimination visible
The X-mark tool records an edge you have ruled out. This matters most on Standard and Challenge boards, where a cell may begin with three or four orthogonal neighbors and several number pairs can repeat across the grid. Marking an impossible edge lowers the candidate counts immediately without committing a domino. You can use pointer drag on touch or mouse input, keyboard controls for navigation, and right-click to erase an existing pair. Undo and Redo preserve both pair placements and X marks, which makes longer deduction chains safer to explore.
Verified Hint prefers a real deduction
Verified Hint first looks for a pair type with one remaining location or an unpaired cell with one remaining partner. When it finds one, it places that exact edge and explains why the move is forced. Only when the current state has no such local forced edge does the helper fall back to one edge from the independently verified unique completion. Using Hint or Check classifies the attempt as Assisted, keeping Clean bests separate. The goal is to make help useful for learning without quietly mixing assisted runs into unassisted records.
Independent uniqueness is stronger than a stored answer
A generated number grid can look plausible while still having two legal domino tilings. V0.130 therefore does not accept a board because the generator remembers the tiling used to create it. The release verifier rebuilds every orthogonally adjacent candidate edge from the visible numbers and solves the exact-cover problem from scratch. A legal completion must cover every cell exactly once and use each unordered pair exactly once. The committed board is admitted only when the independent solver finds exactly one completion and that recovered edge set matches the stored solution. Sixty frozen sources and all eight geometric transforms are checked this way.
Deterministic Practice keeps good puzzles replayable
Practice uses a stable seed in the URL. If you open a Practice board without an s= value, the page adds one with window.history and then keeps that selection stable. The seed chooses a frozen source plus one of eight symmetry transforms, so a shared or bookmarked URL reconstructs the same number grid without contacting a puzzle API. Local autosave stores pairs, X marks, Undo and Redo history, elapsed time, tool choice and Assisted status. Clearing browser storage can remove that local progress, and the product does not pretend it is cloud synced.
Three sizes serve different session lengths
Easy is a 5×4 board using values 0–3 and is designed for a short first solve. Standard is 7×6 using 0–5, which raises the domino inventory to twenty-one types and gives repeated pair appearances more room to interact. Challenge is 8×7 using 0–6, with twenty-eight domino types across fifty-six cells. The same Pair, X-mark, ledger and hint model applies to all three, so increasing difficulty does not require learning a different interface.
Why only two search surfaces
The project intentionally avoids separate thin pages for 5×4 Dominosa, 7×6 Dominosa, hard Dominosa, mobile Dominosa, seed tools, IQ claims or percentile scores. Those labels do not create new workflows. This page answers the general rules, strategy and unlimited-Practice intent, while the Daily Dominosa page answers the returning date-stable challenge intent. Keeping the information architecture restrained reduces duplicate content and keeps each search-facing surface tied to a real product experience.
Continue in the real puzzle
The live game page contains the frozen verified board bank, deterministic selector, Pair and X-mark tools, candidate ledger, Forced move monitor, Undo/Redo, Pause, Check, Verified Hint and browser-local records described here. The explanation is not a substitute for the product.