By clicking in one of the corners of each empty square, you place the blue corner of the tile in that corner, if possible. If it is possible, the situation is indicated by a small blue circle in the corner. You may delete a tile by reclicking it.
The computer may help you. If you click on the bottom line of the applet, he will place all tiles where you do not have any choice after each of your moves. Reclicking the bottom line says you don"t need this.
How many admissible patterns do you think there are?