These rulers if displayed on your screen will measure pixels exactly. Because they are constructed in pixels there is no need to calibrate or adjust for your screen resolution. I use them mostly to allow me to set up a 640x480 or 800x600 browser window to check what my web page will look like at that screen resolution. Much quicker and easier than switching the display to 640x480, then 800x600.

To use them, just save them to your hard drive and display using an image viewer. I use ACDsee because it will give me the image with no unnecessary toolbars etc. To save the images on a PC, right click on each image and select "Save picture as...". There is something similar on a Mac.

The horizontal ruler above is compressed on this screen only — if you save it, it will be the correct size.