The program is a line that appears on your desktop always on top (configurable). You can drag the ruler anywhere on the screen.
If you right-click on the line displays a shortcut menu. It is possible to cause the settings window, change the units of measurement, rotate the ruler 90 degrees, to lock the always-on-top (optional On Top).
Supported the following units: pixels, centimeters, inches, picas, points, and percentages (relative to the length of the line).
In the settings you can change the color of the line, the font size and color.
The disadvantages of the program include the inability to rotate the line by an arbitrary angle.
Installation latest version into Linux Mint:
sudo apt-get install screenruler