Note: this section is a stub. I will add later more information to it.
§α. The Arabic six-vowel system is reconstructed for Proto-Semitic (and even for Proto-Afro-Asiatic by some scholars). This system is generally stable in Arabic. As far as I know, most of the modern dialects of Arabic still distinguish the six vowels of Classical Arabic (with varying phonetic realizations). Two additional vowels ō < *aw and ē < *ay are found in most of the eastern dialects. In the Maghreb these have merged with ū and ī, e.g. Moroccan žūž “two” for Classical zawǵuṋ زَوْجٌ “pair” and ṣȋf “summer” for Classical ṣayfuṋ صَيْفٌ .
§β. In the dialect of Aleppo (and other dialects of western Syria) the vowels i and u have partly merged and become a central vowel ǝ. For example, the Classical bintuṋ بِنْتٌ “daughter” and ʔuxtuṋ أُخْتٌ “sister” are pronounced bən(ə)t and ʔəx(ə)t. However, if the word contains more than one syllable, the vowels i and u are distinguished in the last syllable as e and o. Thus, Classical yanzilu يَنْزِلُ “[he] descends” and yaktubu يَكْتُبُ “[he] writes” are pronounced yənzel <*yinzil and yəktob < *yuktub. The reduced vowel ə is often deleted in unstressed open syllables, e.g. Classical kabȋruṋ كَبِيْرٌ “big” and ṭulȗʕuṋ طُلُوْعٌ “ascending” are pronounced kbīr < *kəbīr < *kibīr and ṭlūʕ < *ṭəlūʕ < *ṭulūʕ.
§γ. Sibawayh talked much about a phenomenon called ʔimȃla·tuṋ الإِمَاْلَةُ “inclination,” which basically means the change ȃ > ȇ. In Sibawayh’s time this change was characteristic of eastern Arabian dialects. Today there are two varieties of ʔimȃla·t in the Arabic dialects, an eastern, conditioned variety (which Sibawayh discussed at length), and a western, unconditioned variety (which Sibawayh dismissed as irregular شَاْذٌّ ). In the conditioned ʔimȃla·t the change ȃ > ȇ happens when ȃ is followed or preceded in the word by a front vowel. This variety of ʔimȃla·t was formerly characteristic of Mesopotamia, but nowadays it seems to be almost extinct there (except perhaps in Mosul). Today the conditioned ʔimȃla·t is characteristic of the dialect of Aleppo; the ʔimȃla·t in this dialect is just like that described by Sibawayh (e.g. sāfar “[he] traveled” vs. ysēfer “[he] travel/will travel;” sāʕad“[he] helped” vs. sēxen “hot;” tamām“perfect” vs. ʔ̣tēl < *qitēl “fighting;” rəᵈžᵈžāl “man” vs. rəᵈžᵈžēli “manly;” məftāħ “key” vs. mfētīħ “keys”). In the unconditioned ʔimȃla·t the change ȃ > ȇ happens without a neighboring front vowel (e.g. nȇs “people,” bȇb “door,” etc.). The unconditioned ʔimȃla·t is widespread in regions that were historically settled by Himyarites, namely in Lebanon, Palestine, Egypt and the Maghreb (historically also in Andalusian Arabic).
§δ. In the dialect of Aleppo the ʔimȃla·t does not work in words borrowed from the Standard language. This (among other factors) has made the ʔimȃla·t phonemic. For example, the word ᵈžihāz (borrowed from Modern Standard Arabic) means “instrument, device,” while the native cognate ᵈžhēz < *ᵈžihēz means the dress and equipment of a bride. The word ktāb < kitāb means “book,” while ktēb < *kitēb means “(contract of) marriage.” The word ᵈžāmʕa·t means “university,” while ᵈžēmʕa is a feminine active participle that means “collecting.” In fact, ʔimȃla·t in Aleppo is not just phonemic but it is also morphemic, since words of the type C₁ēC₂eC₃ are usually active participles, while words of the type C₁āC₂eC₃ are usually not active participles, e.g. kāteb means “writer,” while kēteb is an active participle that means “writing,” ħāres means “guard,” while ħēres is an active participle that means “guarding,” etc. After ē became a morpheme that marks certain lexical classes, it began to be used in improper phonetic environments, e.g. ʔ̣ēʕed < *qēʕid “sitting,” ṭēbex “cooking,” etc.
§ε. In the dialect of Aleppo (and other dialects) ʔimȃla·t regularly affects terminal a vowels that are not preceded by a laryngeal, pharyngeal, velar fricative, or r, e.g. madrase·t “school” for Classical madrasa·tuṋ مَدْرَسَةٌ , lēle·t “night” for layla·tuṋ لَيْلَةٌ , ɣēme·t “cloud” for ɣayma·tuṋ غَيْمَةٌ , ɣȃbe·t “forest” for ɣȃba·tuṋ غَاْبَةٌ ; but šaᵈžra·t “tree” for šaᵈžara·tuṋ شَجَرَةٌ ; warʔ̣a·t “paper” for waraqa·tuṋ وَرَقَةٌ ; ṭabxa·t “cooked meal” for ṭabxa·tuṋ طَبْخَةٌ .
§ζ. The opposite of ʔimȃla·t is tafxȋmuṋ التَّفْخِيْمُ “amplification,” which basically means a backed pronunciation of ȃ. In early Islam this was characteristic of western Arabia. Nowadays it is characteristic of places such as Bahrain, Modern South Arabian (Mehra et al.), the Greater Syrian coast from Cilicia to Tripoli, the Damascus area (and the Iranian plateau). The tafxȋm is a relatively rare phenomenon in the modern dialects and it is not nearly as widespread as ʔimȃla·t.