Rules
You have to find the location of all dominoes on the grid. A domino is a pair of numbers. You can have only one of each pair.
Are you lost? Check out the step by step tutorial

Tap/click to place a domino while in this mode (default)

This button switches to "borders" mode - you can place a red border between numbers which you are certain cannot belong to the same domino.

These 2 buttons allow you to color a certain number (red or blue). Just tap/click on a number while in this mode to color all its occurances.

Zooming
Pinch with two fingers to zoom in/out. If you use a mouse you can use the scroll button.

Moving
Pinch with two fingers and drag to move the puzzle. If you use a mouse you can click and drag with the middle (scroll) button.