No, you cannot pair diagonal tiles in Dominosa — every accepted domino must cover exactly two orthogonally adjacent cells on the 2x3 board. Diagonals sit on a corner-to-corner line that shares only a single point with the next cell, and Dominosa treats any such selection as an invalid move that ends your run after the second mistake. The board itself is a small fixed 2x3 grid showing six number tiles at the start of every run, so the rule is enforced the moment you press Space on a non-adjacent neighbor. This orthogonality rule is the one most readers ask about, because a domino in everyday language can mean any two touching tiles. In Dominosa, the word "domino" keeps the strict mathematical meaning: a 1x2 or 2x1 piece sharing an edge, not a corner. The recursive solver that validates your placements tests exactly the same orthogonality condition, so a diagonal pair is rejected both in the user interface and in the underlying completion search. Once you understand that constraint, the rest of the game becomes a clean exercise in pairing numbers without repeating any unordered pair across three dominoes.
The Orthogonality Rule: Why Diagonal Pairings Are Rejected
The orthogonality rule in Dominosa comes from the standard definition of a domino as a 2x1 or 1x2 rectangle. On a small 2x3 board this leaves four legal horizontal pairings (two in the top row and two in the bottom row) and three legal vertical pairings (one per column). Every other possible two-cell combination is either a diagonal or jumps over an intervening cell, and the interface disables both cases the moment you anchor the first tile. Already covered cells are also disabled, so even a legitimate neighbor becomes unavailable once it has been claimed by an earlier domino.
The rule shows up most clearly when you try to pair the bottom-left "1" with the top-right "1" across the board. Those two cells are diagonal from opposite corners and share no edge; pressing Space on the second tile is rejected as a non-adjacent selection. Two such rejections in a single run freeze the board and mark the game over, so understanding adjacency before you start saves both mistakes and time.
For readers who like to see the same keyboard-first pattern in another tile game, the Color Gradient Puzzle: Play Without Dragging Using Keys guide walks through a similar accessibility-first approach in a different puzzle.
How the Disclosed 2x3 Board Solves Cleanly
The Dominosa board starts every run with the same six numbers visible to the player: 0, 0, 1 on the top row and 1, 2, 2 on the bottom row. Because the board is only 2x3, every legal tiling consists of exactly three dominoes, and the goal is to pick three pairings that cover all six cells without ever repeating an unordered number pair.
The clean solution pairs each vertical column:
- Left column (top 0 with bottom 1) forms the unordered pair 0-1.
- Middle column (top 0 with bottom 2) forms the unordered pair 0-2.
- Right column (top 1 with bottom 2) forms the unordered pair 1-2.
These three value pairs are distinct, which is the only requirement beyond covering every cell. Notice that 0-1 and 1-0 would be the same domino, and 1-2 and 2-1 would also be the same domino — order is never counted. That single property is what makes the puzzle feel solvable: you do not need to remember which end of the pair you started with.
A tempting trap is to start with the horizontal 0-0 pair across the top row. That domino is legal in the adjacency sense, and it uses one of the duplicate numbers on the board. Once you commit it, however, the recursive completion search shows that no valid tiling of the remaining four cells exists, because the leftover values force a repeated unordered pair. The run is flagged as deadlocked the moment the third domino becomes impossible to place, so the horizontal 0-0 opening is a teaching example rather than a working path.
How to Play a Dominosa Run
- Open Dominosa in your browser. The board loads as a fixed 2x3 grid with six numbered tiles; no download or sign-up is required.
- Use the arrow keys to move the outlined cursor to your first tile. The cursor highlights exactly one cell at a time, and you can reach any of the six tiles in a few key presses because the grid is so small.
- Press Space or Enter to anchor the first tile. The selected cell stays highlighted as the active anchor, and every orthogonally adjacent uncovered neighbor becomes a legal second cell.
- Move the cursor to one of those legal neighbors and press Space or Enter again. The two cells lock together as a single domino, and the board records the unordered pair they form (for example, 0-1).
- Repeat the same anchor-and-pair pattern for the remaining cells until all six tiles are covered by exactly three dominoes. Each accepted domino adds 400 points to your score.
- If you change your mind before confirming the second tile, press Space or Enter on the same anchored cell to cancel the selection without spending a move or counting a mistake.
- Press the Restart control at any time to clear every domino, every selection, every mistake, and the running point total so you can start over on the same fresh board.
Mouse and touch users can perform the same actions by selecting the visible cells directly. The interface disables covered cells so they cannot be reused, which prevents accidental over-pairing of an already-claimed tile.
Unique-Pair Validation and Why Order Doesn't Matter
Dominosa uses an unordered pair key for every accepted domino, which means the solver sorts the two numbers before storing them. A 0-1 domino and a 1-0 domino collapse into the same key and therefore count as the same pair. If you ever tried to use both, the second submission would fail validation even though both cells are free and adjacent, because the pair key has already been claimed.
This rule is the heart of the puzzle. On the disclosed board every number repeats — 0 in the top row, 2 in the bottom row, and 1 in opposite corners — so any horizontal opening at the start wastes the 0-0 or 2-2 pair that the rest of the board cannot recover. The vertical column solution avoids that trap entirely by using each value only as part of a distinct unordered pair.
| Selection | Cells | Pair key | Result |
|---|---|---|---|
| Left column | Top 0, Bottom 1 | 0-1 | Accepted, +400 points |
| Middle column | Top 0, Bottom 2 | 0-2 | Accepted, +400 points |
| Right column | Top 1, Bottom 2 | 1-2 | Accepted, +400 points |
| Top row | Top 0, Top 0 | 0-0 | Accepted but leaves no valid completion (deadlock) |
| Top-right to bottom-left | Top 1, Bottom 1 | 1-1 | Rejected — cells are diagonal, not orthogonally adjacent |
The table shows the published verification for the disclosed board. The disclosed three-column solution reaches 1,200 points, and any attempt to use diagonal cells is rejected at the adjacency check before the pair uniqueness check ever runs.
Scoring, Mistakes, and Deadlock Detection
Every accepted domino awards 400 points, and the disclosed board fits exactly three dominoes, so a perfect run finishes at 1,200 points. The score is deterministic: there is no timer, no bonus, and no multiplier. If you restart and play the same three-column solution again, you will score the same 1,200 points.
Mistakes are counted by a strict two-strike rule. A mistake happens whenever you try to confirm a domino whose two cells are not orthogonally adjacent, or whose unordered value pair has already been used by a previous domino. Two mistakes in a single run place the board in a terminal failure state. The interface freezes further selections, the running point total stops moving, and the only way to continue is to press Restart.
Dominosa also checks for logical dead ends after every accepted domino. After a placement, the solver runs an exhaustive completion search that examines the first uncovered cell, tries every uncovered neighbor whose pair value is still unused, and recursively checks the rest. If no perfect unique-pair tiling remains, the run is marked deadlocked immediately so you do not waste more moves on a board that has become unsolvable. This is what makes the horizontal 0-0 opening fail so visibly: the placement is accepted, the score updates by 400, and the very next step shows that no remaining tiling can finish the board.
Best score is saved in local browser storage, so each new run can compete against your previous personal record without needing an account or network request.
Keyboard Controls, Boss Key, and Restart
Dominosa is built keyboard-first, and every action on the board can be performed without a mouse. The arrow keys move the outlined cursor across the six tiles, Space or Enter anchors or confirms a selection, and the same key cancels an anchored tile if it is pressed again before a neighbor is chosen. The Restart control clears the board, the mistakes, and the score, and the shared Boss Key can be triggered by pressing Escape twice to replace the game with a spreadsheet-style view; pressing Escape twice again restores the unchanged board.
| Action | Keyboard | Mouse or touch |
|---|---|---|
| Move cursor | Arrow keys | Click or tap a cell |
| Anchor or confirm a tile | Space or Enter | Click or tap a second cell |
| Cancel an anchored tile | Space or Enter on the same tile | Click the same cell again |
| Restart the run | Restart button | Restart button |
| Boss Key (hide board) | Press Escape twice | Not available |
Because the game relies only on the surface, foreground, line, accent, and danger color tokens, it works under any accessibility setting that the rest of the site supports. There is no canvas, no image asset, no random generator, no timer, no audio, no upload, no account, and no network request, so the entire experience is contained in a single page.
For readers who want to keep practicing compact logic puzzles after finishing a clean Dominosa run, Hashi Bridges, the Numberlink Puzzle, the Hitori Puzzle, and Number Match 10 all share the same small-grid, keyboard-friendly, deterministic design.