British Book Sizes
In printing books, three of the more common sheet sizes have been selected: royal 20 x 25 inches, medium 18 x 23 inches, and crown 15 x 19 inches.
An even number (as 4, 8, 16, 32, 64) of pages is printed on each side of a single large sheet, which is then folded so that the pages are in proper sequence and the outside edges are cut so that the book will open.
The name of the size indicates the fractional part of the sheet one page occupies:
Since the fractional name alone cannot denote an exact size, the name of the sheet size precedes the fractional name. Thus royal octavo is understood to designate a page one-eighth the size of a royal sheet, medium octavo a page one-eighth the size of a medium sheet, and crown octavo a page one-eighth the size of a crown sheet.