Allen and Greenough's New Latin Grammar for Schools and Colleges
J. B. Greenough, G. L. Kittredge, A. A. Howard, Benj. L. D'Ooge, Ed.

Vowel Changes

1. The old diphthong ai became the classical ae (aedīlis for old aidīlis), old oi became oe orū (ūnus for old oinos), and old ou became ū（ dūcō for old doucō ).
In compound verbs the vowel a of the simple verb often appears as i or e, and ae similarly appears asī:—

1. faciō , factum , but cōnficiō , cōnfectum; caedō, but occīdō , and similarly cecīdī , perfect of caedō (cf. cadō,occidō;cecidī, perfect of cadō).

[*] Note.--This change is commonly ascribed to an accentuation on the first syllable, which seems to have been the rule in Latin before the rule given above (see § 12) became established. The original Indo-European accent, however, was not limited by either of these principles; it was probably a musical accent so-called, consisting in a change of pitch, and not merely in a more forcible utterance of the accented syllable.

Two vowels coming together are often contracted:—
1. cōgō for †co-agō; prōmō for †pro-emō;nīl for nihil; dēbeō for †dē-hibed（† dē-habeō ).