[*] 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 vowel1: as, lūc-is ( lūx ), LUC; pāc-is ( pāx ). So in verbs: dūc-ō , ī-s for †eis, from eō , īre; fātur from fārī . [*] c. With reduplication: as, fur-fur, mar-mor , mur-mur. So in verbs: as, gi-gnō (root GEN), si-stō (root STA).
1 The difference in vowel-quantity in the same root (as DŪ˘C) depends on inherited variations (see § 17. a).