Book Sizes

The names of book sizes are based on the old system, still widely used, of considering the size of a page as a fraction of the large sheet of paper on which it was printed. This system is illustrated in Table I below. In printing books, 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. Except for the largest size, the folio, the name of the size indicates the fractional part of the sheet one page occupies (as octavo "eighth"). In this system, 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. But paper is cut into many sheet sizes and even the terms crown, medium, and royal do not always designate sheets of the same dimensions. 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. Actual page sizes run a little smaller than calculations, since the sheets, when folded to page size, are trimmed at top, outside and bottom, the inside edge becoming part of the binding. British sheet size sometimes differs slightly from American.
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  thanks to BookLab II

I've seen these descriptions mentioned before and have never know what the hell they were talking about. Another mystery solved.