The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
94. Most nouns of the Fourth Declension are formed
from verb-stems, or roots, by means of the suffix -tus (-sus) (§ 238. b):
- cantus, song, CAN,
canó, sing; cásus (for cad-tus), chance, CAD, cadó, fall; exsulátus, exile, from exsuló, to be an exile (exsul).
a. Many are formed either from verb-stems
not in use, or by analogy:
- cónsulátus (as if from cónsuló, -áre), senátus,
b. The accusative and the dative or
ablative of nouns in -tus (-sus) form
the Supines of verbs (§ 159. b): as, spectátum, petítum; dictú,
c. Of many verbal derivatives only the
ablative is used as a noun: as, iussú
(meó), by (my) command; so iniussú (populí), without (the people's) order. Of some only the dative is used: as, dívísuí.