Crown (Paper)
Modern equivalent
20 × 15 in = 300 sq in = 508 × 381 mm
A 20 by 15 inch sheet, and the base of the only traditional British paper series that doubles exactly.
Crown is the one traditional British sheet whose series behaves itself arithmetically. A crown is 20 by 15 inches, or 300 square inches. Double crown is 30 by 20, which is 600 exactly. Quad crown is 40 by 30, which is 1,200. Double quad crown is 60 by 40.
That clean doubling is why the crown series outlived most of the others. Printers could plan a job across sheet sizes without recalculating, and the proportions survive in British poster and display printing, where double crown remains a recognised size.
It anticipates the logic of the A series by several centuries, though imperfectly: the A sizes preserve their proportions when halved, where the crown series alternates between two shapes as it doubles. Right principle, rougher execution.
Like the other traditional sizes, the name is thought to come from the watermark used by the mills that made it, though we have not found that documented in a source worth citing.