"Old World quail" is a collective term for several unrelated genera of smaller galliform species under the tribe Coturnicini of the subfamily Phasianinae. Note that although species which are commonly referred to as "Old World quail" fall in the same tribe, they are paraphyletic with respect to the other genera of the tribe, such as Alectoris, Tetraogallus, Ammoperdix, Margaroperdix, and Pternistis. New World quail are also housed in the order Galliformes, but they are classified under a different family, Odontophoridae.
The buttonquails or hemipodes, despite their common name and similar resemblance to smaller members like the Asian blue quail (erroneously called "button quail"), are not related at all. They are presently placed in the family Turnicidae of the order Charadriiformes, in the same category as shorebirds.