Classical Adûnaic had three vowels [a], [i] and [u] that could be either short of long, descendants of the Primitive Adûnaic vowels (SD/422). It also had two vowels [ē] and [ō] which could only be long, derived from earlier short diphthongs [ai] and [au] (SD/423). As a rule, classical Adûnaic did not tolerate vowels in hiatus (in contact without forming a diphthong). See the rules for Adûnaic vowel-combinations for further discussion. Adûnaic could also have over-long vowels produced by contact between two long vowels, but it is not clear whether they were distinguished from long vowels by the time of Classical Adûnaic (SD/424-5).