It is a grid ruler that can measure the length on the PC screen.
When you click on the grid rectangle appears.
First, click on the Setting button, please the initial configuration of the ruler.
When you fix up what measure on the screen, please note that as not to damage the screen.
So that the blue square is the same size as that of the card, please adjust the slide bar.
dpi
You can fine-tune the arrow keys on the keyboard.