The automatic border that Paul discusses, generated by AutoFormat as you type, will go from paragraph left indent to paragraph right indent. Usually this is from left to right margin, but indents can be different from the margins.


Margins and Indents in Word

The line shown in your question would normally be such a border going from indent to indent.

Thanks, JimP, macropod, and Charles Kenyon. I will try each suggestion and use what best fits my need.

Paul, typing "---" and pressing enter just gives me three hyphens--no automatic conversion. Also, how do I reach the border tools?

Thanks.

Paragraph borders can be reached through the paragraph group on the Home tab.
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What Paul was referring to was the automatic application of borders through the feature "Autoformat as you type." This can be reached through File > Options > Proofing > AutoCorrect Options. These options are checked by default in an initial installation of Word. I, and many experienced Word users, turn most of them off.
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The reason we turn them off is that they can be a real pain if the user doesn't understand what is happening. For the experienced user, it is simpler to use the actual controls. See:

• Weird Lines and Borders
• Troublesome Lines

Thanks, all. The paragraph bottom border works best for me.