This is a very useful tool to use if you would like to measure elements. This function is located as a toolbar button and also under the View Menu. If you would like to change the units of measure, you can do that by going to the Tools Menu, selecting Options, clicking on the Measurement Units and look for Ruler in the list.
To use this feature click the ruler toolbar button , click on the starting point from which you would like to measure and drag your mouse to end point of your measurement. While doing this look to the bottom of your screen on the status bar to view the measurement. Let go of your mouse button to start a new measurement. To turn off the Ruler feature, click on the ruler toolbar button again.
The keyboard shortcut key for this is <Alt> R.