[2466.] Hic, illic and istic, when adverbs, have a long final syllable; but when nominative singular masculine, have the final syllable regularly short.
[2467.] In Plautus frūstrā always where determinable (seven times) has the final syllable short. contrā sometimes has a short final syllable in old Latin.
[2468.] In Latin poetry down to the time of Cicero, final s often does not “make position” before a following consonant ([66]); as, tempŭs fert (Plaut.); magĭs stetisse (Ter.).
[2469.] The first syllable of ille, illic (the pronoun), quippe, immō, inde, unde, nempe, omnis, and perhaps iste, is sometimes shortened.
In ille, illic, quippe, and immō the shortening is, some hold, due to the fact that in common speech one of the double consonants was often pronounced faintly or not at all; while in inde, unde, nempe, and omnis the nasal was very faintly sounded before the following consonant. But some authorities hold that always in nempe, and sometimes in ille, quippe, inde, unde, and perhaps iste, before an initial consonant final e disappears, and the word becomes a monosyllable.
[Law of Iambic Shortening.]
[2470.] A long syllable, preceded by a short monosyllable or by a short initial syllable, and immediately preceded or followed by the verse-ictus, may be shortened: as, ét hŭnc, dómŏ mē, ad ŭxṓrem, volŭntā́te.
The short monosyllable may be a word which has become monosyllabic by elision: as, ég(o) hănc.
[2471.] If the syllable to be shortened is the first of a word of more than one syllable, or the second of a polysyllable, it must be one which is long by position, not by nature. There are some possible exceptions to this rule, such as verĕbā́minī (T. Ph. 902); but these are few and doubtful.
[2472.] Iambic shortening took place not only in verse, but also to a considerable extent in common speech, particularly in iambic words (see [130]), in which the accent coöperated with the verse-ictus to produce the shortening.