[132]. A long vowel is regularly shortened, in the classical period, before final -t and -m and, in words of more than one syllable, also before final r and l.
Thus, soror, sister, for Plautus’s sorōr, from the stem sorōr- ([487]); ūtar, I may use, for Plautus’s ūtār (cf. ūtāris); bacchanal for Plautus’s bacchanāl; animal, exemplar from the stems animāl- ([530]) and exemplār- ([537]); but the long vowel is retained in the monosyllables fūr, thief, sōl, sun; pōnēbat, he placed, for Plautus’s pōnēbāt (cf. pōnēbās); iūbet, he commanded, for Plautus’s iūbēt; eram, I was, but erās; rēxerim, I may have ruled, but rēxerīs ([877]); -um in the genitive plural of -o- stems is for -ūm ([462]); mēnsam, table, for *mēnsām from the stem mensā-; rem, from rē- (rēs), spem from spē- (spēs).
[133]. (1.) In a few cases the length of the vowel has been transferred to the following consonant, the length of which is then indicated by doubling it ([81]): as, littera for lītera, LEITERAS; Iuppiter for Iūpiter; parricīda for pāri-cīda, murder of a member of the same clan (*pāro-, member of a clan, Doric πᾶός, a relative); cuppa for cūpa, barrel. The legal formula sī pāret, if it appear, was vulgarly pronounced sī parret (Festus).
([2.]) Since the doubled unsyllabic i (i̭) between vowels ([23]; [166, 9]; [153, 2]) is commonly written single, the vowel preceding it is often erroneously marked long: as, āiō wrongly for aiō, i.e. ai̭i̭ō, I say; māior wrongly for maior, i.e. mai̭i̭or, greater; pēior wrongly for peior, i.e. pei̭i̭or, worse; ēius, of him, cūius, of whom, hūius, of him, all wrongly for eius, cuius, huius i.e. ei̭i̭us, cui̭i̭us, huii̭us ([153, 2]). In all these words the first syllable was long but not the vowel.
[134]. (1.) In some foreign proper names and in a very few Latin words the quantity of a vowel varied. Vergil has Sȳchaeus and Sychaeus within six verses; also Āsia and Asia, Lavīnium and Lāvīnius; so also glōmus (Lucr.), glomus (Hor.); cōturnīx (Plaut., Lucr.), coturnīx (Ov.).
([2.]) Sometimes such variations in vowel quantity are only apparent: thus, the occasional long final -ē of the active infinitive (darē, prōmerē) has probably a different origin from the usual -ĕ. For metrical lengthening, see [2505].
[QUANTITATIVE VOWEL GRADATION.]
[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.