The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
58. Most mute stems are Masculine or Feminine. Those
that are neuter have for the Nominative the simple stem. But, -
a. Lingual stems (t, d) ending in two consonants drop the final
mute: as, cor (stem cord-), lac
(stem lact-). So stems in at- from the Greek: as, poéma (stem poémat-).
b. The stem capit- shows u in the nominative (caput for ¥capot).