Tile the rectangle with dominoes (1×2 rectangles) so that every possible domino appears exactly once (that is, every possible pair of numbers, including doubles).
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click).
Click a pair of numbers to place or remove a domino covering them. Right-click it to place or remove a line between them, if you think a domino definitely cannot go there. Keep the button down and drag to do the same to every pair of numbers you pass over that looked the same as the first.
With the keyboard, the arrow keys move a cursor around the board. Enter does what a click does to the pair of numbers under it, and Space what a right-click does.
Aim a click between the two numbers, nearer their shared edge than the middle of either; the keyboard’s cursor moves by half a square, so it rests on those edges. Dominoes light up red if two identical ones appear on the grid.
A new domino replaces any dominoes it overlaps, and clears the lines around it. A line can only go between two squares that are not part of a domino.
Right-click the middle of a square, or type a number, to highlight every square showing that number; up to two numbers can be highlighted at once, and doing it again turns the highlight off.
The reference panel lists every domino in the set, marking the ones you have placed and flagging any placed twice. Pick one to highlight where on the board it could still go.
Hint explains the next step rather than simply making it. It reasons from the dominoes you have placed, so it carries on from wherever you are, as long as none of them is wrong; if one is, it asks you to fix the highlighted mistakes first. It does not rely on the lines you have drawn between numbers: it works each one out for itself, and leaves out a step whose line is already on the board. A spot, in its words, is a pair of neighboring squares a domino could cover.
When one reason rules out several spots, the hint shows them one after another, saying “for the same reason” rather than repeating it.
A few ideas are worth learning by name:
On an Unreasonable board the hint may stop and say that nothing further follows by deduction.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.