duodecimo
Concise Oxford English Dictionary © 2008 Oxford University Press:
duodecimo /ˌdjuːə(ʊ)ˈdɛsɪməʊ/
▶noun (pl. duodecimos) a size of book in which each leaf is one twelfth of the size of the printing sheet.
– origin C17: from L. (in) duodecimo ‘in a twelfth’.
'duodecimo' also found in these Oxford entries:

