LaTeX/Rules and Struts

Rules
The command in normal use produces a simple black box:

The parameter determines the height, whereas  determines the width  of the produced rule. With the optional parameter, you can optionally raise or lower the produced rule above or below the baseline.

Here is an example (the thin lines are located at the baseline):

This is useful for drawing vertical and horizontal lines.

Struts
A special case is a rule with no width but a certain height. In professional typesetting, this is called a strut. It is used to guarantee that an element on a page has a certain minimal height. You could use it in a tabular environment or in boxes to make sure a row has a certain minimum height.

In LaTeX a strut is defined as

Stretched rules
LaTeX provides the command, which work like a stretched horizontal space. See the Lengths chapter.