In printing a rule is a line, though of course there are also rules in the conventional sense that printers follow. Think for example of Hart’s Rules for Compositors and Readers at the University Press Oxford. Although the book doesn’t contain rulesª directly about rules, it does include plenty of reference in its rulesª to the appropriate use of rules, for instance in its rulesª about setting rules in tables.

Rules, in their manifestation as straight lines, are measured in points or fractions thereof. We tend to talk of the weight of a rule, not its thickness. The rules at the top of this page might be 1 point rules “printing” in 50% black, though translating from screen to print is nonsensical because of variable rendering. As a ruleª these rules will look different on your iPhone, than they do on your iPad, or on your desktop computer.

One should bear in mind the related manifestation of rule: this is an en rule – and this an em rule —. In well-mannered typesetting these will be set with a word space on either side of them. This may be a British ruleª rather than an American one though.

Of course the one ruler will rule them all, which hints at the derivation of the printer’s straight line. After all printers have other uses for the word line. The OED informs us that the phrase “rule and line” means determined or regulated, rigid or strict. Apparently the word rule used also to refer to riotous conduct especially in north England and Scotland, where we love such activity.