The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
235. Both primary and secondary suffixes, especially
in the form of compound suffixes, were used in Latin with more or less
consciousness of their meaning. They may therefore be called Significant
They form: (1) Nouns of Agency; (2) Abstract Nouns (including Names of Actions); (3) Adjectives (active or passive).
NOTE: There is really no difference in etymology between an
adjective and a noun, except that some formations are habitually used as
adjectives and others as nouns (§ 20. b. N. 2).