Glossary of Printing Terms:
Rule

1. A straight line used as a graphic element.

2. The thickness of the line is measured in points.

3. A continuous line, thick or thin, that is used in decorative printing.

In the 1800s and early 1900s, title pages were often enclosed in plain rule-borders.

The term can also apply to a decorative line on a binding, which may be in blind or gilt.