Dominosa

Overview

Dominosa hides a complete set of dominoes inside a rectangle of numbers. Your job is to recover the borders between the dominoes by pairing neighbouring squares. Each pair of numbers may appear only once, regardless of order: a two beside a five represents the same domino as a five beside a two. Every square must belong to exactly one pair. Unlike a matching game, the numbers stay where they are and there is no tile supply to drag onto the board. The challenge comes from competition between possible placements of the same pair. Marking one domino can eliminate several alternatives elsewhere. Different board sizes in the Type menu change the size of the domino set and the amount of bookkeeping required.

How to Play

Click the boundary between two orthogonally adjacent numbers to join them into a domino. Click it again to remove that choice. A secondary click marks a boundary that you believe must separate dominoes. On a phone, enable Secondary before tapping an exclusion boundary, then turn it off to resume pairing. Continue until the rectangle is fully covered without a repeated unordered pair. Undo and Redo help compare placements, while Restart clears the current attempt. The engine checks the completed arrangement.

Tips & Strategy

Search first for a number pair that appears in only one possible location. That placement is forced even if its surrounding squares look ambiguous. Then inspect corners and cells whose neighbours have mostly been claimed; a cell with only one available partner gives another forced domino. Remember to eliminate both orientations of a pair after placing it. Do not simply join equal adjacent numbers because they look conspicuous: the matching double may be forced somewhere else. If a choice strands a single square between completed dominoes, undo before building a longer chain on top of it.

About this version

Dominosa uses the locally compiled dominosa engine from Simon Tatham’s Portable Puzzle Collection. The original authors retain credit under the MIT license. SonoTap provides the browser adaptation and this guide.