rule
A thin plate of brass or other metal, of the same height as the type, and used for printing lines, as between columns on the same page, or in tabular work.
rule: metal strip that prints lines between columns
In letterpress printing, a rule is a thin metal bar, typically brass or steel, cast or machined to the exact height of type (0.918 inches in Anglo-American standard). It prints as a solid line when locked into the forme alongside type, serving the mechanical function of dividing space on the page. Rules come in various thicknesses: hair rules measure 0.5 points, while common rules range from 1 to 4 points, with heavier ornamental rules reaching 12 points or more.
The primary work of rules appears in tabular composition and column separation. A printer would lock vertical rules between columns of text to create visual boundaries; horizontal rules would separate rows of figures or table headings from body matter. Because rules are cast to type height, they lock flush into the forme and print in a single pass with the surrounding type, avoiding the need for separate runs or overlay techniques. This mechanical simplicity made rules indispensable to newspaper and book production.
Rules exhibit wear and damage patterns distinct from type itself. The edges of a much-used rule flatten and develop microscopic burrs that can cause uneven inking or faint shadow lines on the printed sheet. Corrosion, particularly in brass rules exposed to damp conditions or acidic paper residue, results in pitting that shows as speckling in the printed line. A bent rule cannot be locked tightly into the forme and will either print faintly or damage adjacent type during the lock-up process.
Variants and ornamental use
Beyond the plain rule, printers used decorative variants including rules with small serifs or rounded edges, and what are called leads with rules, which combined a thin rule on top of a lead spacer (typically 6 or 12 points tall) to create ruled lines at specific distances below text. Printers also cast ornamental borders and corner pieces from similar metal stock, though these are technically distinct from rules.
The term rule derives from the Latin regula, meaning a straight line or standard measure. In the hand-composing and machine-composing era of printing, rules remained essential until phototypesetting and offset lithography displaced letterpress. Modern digital prepress has rendered physical rules obsolete, though the term survives in page-layout software as a generic reference to drawn lines.