Allen and Greenough's New Latin Grammar, section 231

Heavy Construction

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231.

A root used as a stem may appear -

a. With a short vowel: as, duc-is (dux), DUC; nec-is (nex); i-s, i-d. So in verbs: as, es-t, fer-t (cf. § 174. 2).

b. With a long vowel[1][The difference in vowel-quantity in the same root (as DUC) depends on inherited variations (see § 17. a).]: as, lúc-is (lúx), LUC; pác-is (páx). So in verbs: dúc-ó, í-s for eis, from eó, íre; fátur from fárí.

c. With reduplication: as, fur-fur, mar-mor, mur-mur. So in verbs: as, gi-gnó (root GEN), si-stó (root STA).