Definition
Pica
A pica is a printing unit equal to 12 points, or 1/6 inch. Page measures and margins are traditionally specified in picas — a 27-pica measure is 4.5 inches.
Picas survive because they divide neatly by the units type is already built from: a 12 pt line of type is exactly one pica tall, so a page laid out in picas keeps its baselines on a whole-number grid. Self-publishing platforms ask for inches or millimetres, so most authors never meet the unit — but typesetting software still speaks it, and a spec sheet quoting 27 × 45 picas is describing a page area, not a mistake.
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