108. Diphthongs, whether medial or final, are treated alike in atonic
syllables.
(_a._) Atonic #ei#, #oi#, and #ai# (#ae#) became #ī#: as, #lupī#,
_wolves_ (nom. pl.), for #*lupoi# (Gr. λύκοι); #bellī#, _in war_ (loc.
sg., 460, 1338), for #*bellei# (Greek οἴκει) or #*belloi# (Greek οἴκοι);
#éxīstimō#, _I consider_, from #aestimō#; #cóncīdō#, _I strike down_,
from #caedō#; Cicero, _O._ 159, mentions #inīcum#, _unfair_, for
#*ínaecum#, and #concīsum# for #*cóncaesum#; so also, probably, #hīc#,
_this_, arose from #hoic# (662) when used as a proclitic (92).
(_b._) Atonic #ou# and #au# became #ū#: as, #ínclūdō#, _I include_, from
#claudō#; #áccūsāre#, _to accuse_, from #causa#.
109. There are not a few cases in which the atonic vowel does not
conform to the rules given above (102-108). These are usually compounds
which show the vowel of the simple verb. Some of these were formed at a
time when the early recessive accent was no longer in force and
consequently there was no cause for weakening; in others the vowel of
the simple verb was by analogy substituted for the weakened vowel of the
compound: as, #appetō#, _I strive after_, from #petō#, which ought to
have #i# like #colligō#, _collect_, from #legō#; #intermedius#,
_intermediate_, but #dīmidius#, _half_; #dēfraudāre#, _to cheat_, by the
side of #dēfrūdāre# from #fraudāre#; instead of the common #redarguō#,
_I refute_, Scipio Africanus minor Pauli filius (185-129 B.C.) said
#rederguō#, and #pertīsum# for #pertaesum#, but both Cicero (_O._ 159)
and Lucilius discountenance #pertīsum# as the sign of a pedantic prig.
In a few cases the reverse process took place, and the weakened vowel
which arose in the compound was transferred to the simple verb: as,
#clūdō#, _I close_ (958), for #claudō#, which owes its #ū# to compounds
like #occlūdō#. For a case where the vowel of the preceding syllable
acted as a stay to the expected change, see 104, _d_.