a. Forms from the same stem have the same quantity: as, amó, amávistí; genus, generis.
Exceptions. - 1. bós, lár, más, pár, pés, sál, - also arbós, - have a long vowel in the nominative, though the stem-vowel is short (cf. genitive bovis etc.).
[The quantity of the stem-vowel may be seen in the genitive singular.]
2. Nouns in -or, genitive -óris, have the vowel shortened before the final r: as, honor. (But this shortening is comparatively late, so that in early Latin these nominatives are often found long.)
3. Verb-forms with vowel originally long regularly shorten it before final m, r, or t: as, amem, amer, dícerer, amet (compare amémus), díceret, audit, fit.
NOTE: The final syllable in t of the perfect was long in old Latin, but is short in the classic period.
4. A few long stem-syllables are shortened: as, ácer, acerbus. So dé-ieró and pé-ieró, weakened from iúró.
b. Forms from the same root often show inherited variations of vowel quantity (see § 17): as, dícó (cf. maledicus); dúcó (dux, ducis); fídó (perfidus) vóx, vócis (vocó); léx, légis (legó).
c. Compounds retain the quantity of the words which compose them as, oc-cidó (cadó), oc-cídó (caedó), in-íquus (aequus).
NOTE: Greek words compounded with