Within the book trade, a book's size is usually specified
by the number of folds made in the large pieces of paper
on which the book is printed (briefly, a book is printed on
large sheets of paper which are folded, bound
and then trimmed. The historical reason books were
printed on a couple of large sheets of paper rather than
a lot of small sheets was that it was cheaper.)
TABLE OF BOOK SIZES
name height width
---------------------------------------------
folio 19 12
quarto 12 9.5
octavo 9 6
duodecimo (twelvemo) 7.75 5
sextodecimo (sixteenmo) 6.75 4
N.B.: The height and width measurements are given in inches.