The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
102. Some nouns of the above classes (§ 101. 1 - 4), have a corresponding singular, as noun or adjective, often in a
1. As noun, to denote a single object: as, Bacchánal, a spot sacred to
Bacchus; optimás, an
2. As adjective: as, Cató
Maior, Cato the Elder.
3. In a sense rare, or found only in early Latin: as, scála, a ladder; valva, a
door; artus, a joint.