composing rule
A thin slip of brass or steel, against which the type is arranged in a composing stick, or by the aid of which stickfuls or handfuls or type are lifted.
composing rule: the brass guide that holds type in place
A composing rule is a flat, rectangular strip of brass or steel, typically 0.015 to 0.02 inches thick, that serves as the bottom boundary of a composing stick during hand typesetting. The printer places the rule at the base of the stick, then builds lines of type against it, character by character. The rule itself does not print; it is a precision spacer and alignment device that ensures every line of type sits at exactly the same height and angle.
The rule's length matches the width of the desired line measure (column width). Standard lengths range from 12 picas to 48 picas or more, depending on the publication format. The rule must be straight and free of burrs, as any deviation will cause the finished line of type to sit uneven on the printing bed. Printers keep a set of rules in different lengths to accommodate different column widths without having to adjust the stick itself.
Practical use and variants
When a composing stick is filled with one or more lines of type, the compositor slides the composing rule under the completed lines, lifts the entire bundle, and slides it out onto a galley or receiving surface. The rule prevents the type from scattering and distributes the lifting force evenly across the line. Some rules have a slight hook or lip at one end to aid in gripping. Brass rules are preferred over steel because they are less likely to damage type when slipped underneath; they also resist corrosion from ink and handling oils better than plain steel.
The composing rule is sometimes confused with the setting rule, which is placed on top of the type to hold it down during composition. The composing rule works from below; the setting rule works from above. A compositor needs both: the setting rule to keep the type from popping out during the character-by-character placement, and the composing rule to support and transfer the finished lines.
In large composing rooms, rules become worn, warped, or bent from constant use and are periodically straightened or replaced. A rule that has lost its true flatness will introduce a taper into the printed line, making text appear to lean slightly. Because the rule is in direct contact with thousands of pieces of type over its working life, it is considered a consumable tool, though a good brass rule can serve for years if maintained with care.