The earliest forms of Arabic are known as Old Arabic and survive in inscriptions in Ancient North Arabian scripts as well as fragments of pre-Islamic poetry preserved in the classical literature. By the late 6th century AD, it is hypothesized that 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". 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".