(i.) The quantity of the final syllable had no effect on accent.
(ii.) If the penult was long, it bore the accent (amābấmus).
(iii.) If the penult was short, then
(a) if the ante-penult was long, it bore the accent (amấbimus);
(b) if the ante-penult was short, then
(i.) if the ante-ante-penult was long, the accent was on the ante-penult (amīcítia); but
(ii.) if the ante-ante-penult was also short, it bore the accent (cólumine, puéritia).
Exon’s Laws of Syncope.—With these facts are now linked what may be called Exon’s Laws, viz:—
In pre-Plautine Latin in all words or word-groups of four or more syllables whose chief accent is on one long syllable, a short unaccented medial vowel was syncopated; thus *quínquedecem became *quínqdecem and thence quíndecim (for the -im see § 19), *súps-emere became *súpsmere and that sūmere (on -psm- v. inf.) *súrregere, *surregémus, and the like became surgere, surgémus, and the rest of the paradigm followed; so probably validé bonus became valdé bonus, exterá viam became extrá viam; so *supo-téndo became subtendo (pronounced sup-tendo), *āridére, *avidére (from āridus, avidus) became ārdére, audére. But the influence of cognate forms often interfered; posterí-diē became postrídiē, but in posterórum, posterárum the short syllable was restored by the influence of the trisyllabic cases, pósterus, pósterī, &c., to which the law did not apply. Conversely, the nom. *áridor (more correctly at this period *āridōs), which would not have been contracted, followed the form of ārdórem (from *āridórem), ārdére, &c.
The same change produced the monosyllabic forms nec, ac, neu, seu, from neque, &c., before consonants, since they had no accent of their own, but were always pronounced in one breath with the following word, neque tántum becoming nec tantum, and the like. So in Plautus (and probably always in spoken Latin) the words nemp(e), ind(e), quipp(e), ill(e), are regularly monosyllables.