The usual domino set has dominoes, (0,0), (0,1), ..., (6,6) for a total of 28 dominoes. (The (0,1) domino is the same as the (1,0) domino, etc.) Another domino set has dominoes from (0,0) to (9,9) for a total of 55 dominoes. The opening panel allows the user to choose the size of the domino set by selecting the largest domino number. The default has been chosen as 6, but you can replace this by a number between 3 and 9 inclusive.
After you choose the size of the domino set (say you enter 6 and press OK), then you see on the left of the screen a rectangular array that is formed from these dominoes. However, the actual placement of the dominoes has not been recorded. Your assignment, should you choose to accept it, is to find where each domino occurs in the array. There may be many places in the array with 2 next to 3, but only one of them can be the domino (2,3). After you decide which, you simply outline that domino by clicking on its edges. Clicking on an edge turns that edge to black. If you decide you made a mistake, clicking on it again turns the edge back to white. Pressing the "reset" button changes all edges back to white.
If there is a unique place in the array with a 2 next to a 3, that must be the (2,3) domino. Since it may be tedious to find how many times an x occurs next to a y in the array, there is placed on the right of the screen a table giving for each domino how many possible placements for that domino remain. This speeds up the detective work.
You win if you outline all the dominoes "correctly", in the sense that you have outlined the dominoes and each domino occurs once and only once in the array. If this is the case, you are told and congratulated.
Pressing the "new game" button reshuffles the dominoes into a new array. If you want to change the size of the domino set, just click the refresh button of your browser.