It is hypothesized that by the late 6th century AD a relatively uniform intertribal "poetic koiné", a synthetic language distinct from the spoken vernaculars, had developed with conservative as well as innovative features, including the case endings known as ʾiʿrab.

[5] It is uncertain to what degree the spoken vernaculars corresponded to the literary style, however, as many surviving inscriptions in the region seem to indicate simplification or absence of the inflectional morphology of Classical Arabic.

It is often said that the Bedouin dialects of Najd were probably the most conservative (or at least resembled the elevated intertribal idiom morphologically and lexically more than the other contemporary vernaculars), a view possibly supported by the romanticization of the ‘purity’ of the language of the desert-dwellers (as opposed to the "corrupted" dialects of the city-dwellers) expressed in many medieval Arabic works, especially those on grammar, though some argue that all the spoken vernaculars probably deviated greatly from the supraregional literary norm to different degrees, while others, such as Joshua Blau, believe that "the differences between the classical and spoken language were not too far-reaching".

[7] By the 2nd century AH (9th century AD/CE) the language had been standardized by Arabic grammarians and knowledge of Classical Arabic became a prerequisite for rising into the higher classes throughout the Islamic world, since it was the lingua franca across the Middle East, North Africa, and the Horn of Africa, and thus the region eventually developed into a widespread state of diglossia.

Thus, exegetes, theologians, and grammarians who entertained the idea of the presence of "impurities" (for example, naturalized loanwords) in the Qur'an were severely criticized and their proposed etymologies denounced in most cases.

[note 4] Poems and sayings attributed to Arabic-speaking personages who lived before the standardization of the Classical idiom, which are preserved mainly in far later manuscripts, contain traces of elements in morphology and syntax that began to be regarded as chiefly poetic or characteristically regional or dialectal.

Note the inclusion of palatal /ɕ/, which alone among the palatal consonants exhibits assimilation, indicating that assimilation ceased to be productive before that consonant shifted from Old Arabic /ɬ/: Proto-Central Semitic, Proto-Arabic, various forms of Old Arabic, and some modern Najdi dialects to this day have alternation in the performative vowel of the prefix conjugation, depending on the stem vowel of the verb.