Tutoring chemistry, naming acids might arise. The tutor brings up a time to use hypo in naming an acid.
From a simple point of view, the hypo acid has one less oxygen than the -ous acid. For example:
chlorous acid: HClO2
hypochlorous acid: HOCl
Mortimer, Charles E. Chemistry, sixth ed. Belmont: Wadsworth, 1986.
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.