Rule

A drawn line used to divide or underline. The quietest mark in design, and the easiest to overuse.

A rule is a drawn line with a job: divide this from that, underline this claim, carry the eye across a gap. The hairlines threading these pages are rules at their quietest, marking sections at one pixel so structure is felt rather than announced.

Its danger is cheapness. A rule is the easiest order a layout can buy, so weak layouts buy too many, fencing every element until the page is a form. Before drawing one we ask whether spacing alone could make the separation. If it can, the rule is clutter with a straight edge.