The ruler in Microsoft Word runs horizontally along the top of your workspace and helps you align the elements in your document correctly.
If you can’t see a ruler, go to View tab and make sure that Ruler is checked (in the Show group):
But the numbers that are displayed on the ruler – what do they refer to? That is, what units of measure does the ruler use? To determine which units you are using, or to change them, click the File tab > Options, and click on the Advanced tab on the left. In the Display section, you should see the Show measurements in units of dropdown selector.
This shows you the units currently being used, and also allows you to change them. You’ll find that the most popular units are inches and centimeters, but the whole list of options is here:
Knowing the units of measure being used is helpful when you are dealing with margins, for example. In the image below, you can tell at a glance that the left margin is set to be 2.5 cm, if you know that your units of measure are centimeters.