135. The same stem often shows a long vowel in some of its forms and a
short vowel in others. In most cases these variations of quantity were
not developed on Latin soil but inherited from a much earlier period.
Such old inherited differences in vowel quantity are called
_quantitative vowel gradation_.
(1.) Instances of this are #prō# for #*prōd# (149; cf. #prōdesse#) and
#pro-# (Greek πρό); #nē# and #ne-# in #nescius#; the couples #regō#, _I
rule_, and #rēxī#; #vehō#, _I draw_, #vēxī#; #veniō#, _I come_, #vēnī#,
where the long vowel is characteristic of the perfect stem (862);
#vocō#, _I call_, and #vōx#, _voice_; #regō#, _I rule_, and #rēx#,
_ruler_; #legō#, _I read_, and #lēx#, _bill_; #sedeō#, _I sit_, and
#sēdēs#, _seat_; #fidēs#, _confidence_, and #fīdō#, _I trust_; #dux#
(cf. #ducis#), _leader_, and #dūcō#, _I lead_, where verb and noun are
differentiated by the quantity of the root vowel; and many others.
(2.) Sometimes the reduction of the vowel in certain forms amounts to
complete loss, as in the adverbial ending #-is-# in #magis# (346, 363)
compared with the comparative suffix #-ios#, #-iōs# (Nom. #-ior#, Genit.
#-iōris#); in the oblique cases of the stem #carōn-# (nomin. sg. #carō#,
497), where the suffix becomes #-n-# (545), genitive #car-n-is#; in the
suffix #-ter#, which becomes #-tr-# in all cases but the nom. sg.
(#pater#, #patris#, etc., 470, 487); in the feminine #-tr-ī-c-# to the
suffix #-tor-#; but the nom. sing. #Caecīlis# (465) for #Caecīlios# is
probably due to syncope.