The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
229. Stems are either identical with roots or
derived from them. They are of two classes: (1) Noun-stems (including Adjective-stems) and (2) Verb-stems.
NOTE: Noun-stems and verb-stems were not originally different
(see p.163), and in the consciousness of the Romans were often confounded;
but in general they were treated as distinct.