with Aramaic elements. Under Arsacid hegemony, this Aramaic-derived writing system for Iranian languages came to be associated with the Parthians in particular (it may have originated in the Parthian chancellories), and thus the writing system came to be called pahlavi "Parthian" too. Aside from Parthian, Aramaic-derived writing was adopted for at least four other Middle Iranian languages, one of which was Middle Persian. In the 3rd-century CE, the Parthian Arsacids were overthrown by the Sassanids, who were natives of the south-west and thus spoke Middle Persian as their native language. Under Sassanid hegemony, the Middle Persian language became a prestige dialect and thus also came to be used by non-Persian Iranians. In the 7th-century, the Sassanids were overthrown by the Arabs. Under Arab influence, Iranian languages began to be written in Arabic script (adapted to Iranian phonology), while Middle Persian began to rapidly evolve into New Persian and the name parsik became Arabicized farsi. Not all Iranians were comfortable with these Arabic-influenced developments, in particular, members of the literate elite, which in Sassanid times consisted primarily of Zoroastrian priests. Those former elites vigorously rejected what they perceived as 'Un-Iranian', and continued to use the "old" language (i.e. Middle Persian) and Aramaic-derived writing system. In time, the name of the writing system, pahlavi "Parthian", began to be applied to the "old" Middle Persian language as well, thus distinguishing it from the "new" language, farsi. Consequently, 'pahlavi' came to denote the particularly Zoroastrian, exclusively written, late form of Middle Persian. Since almost all surviving Middle Persian literature is in this particular late form of exclusively written Zoroastrian Middle Persian, in popular imagination the term 'Pahlavi' became synonymous with Middle Persian itself. The ISO 639 language code for Middle Persian is pal, which reflects the post-Sasanian era use of the term Pahlavi to refer to the language and not only the script. Transition from Old Persian In the classification of the Iranian languages, the Middle Period includes those languages which were common in Iran from the fall of the Achaemenid Empire in the fourth century BCE up to the fall of the Sasanian Empire in the seventh century CE. The most important and distinct development in the structure of Iranian languages of this period is the transformation from the synthetic form of the Old Period (Old Persian and Avestan) to an analytic form: nouns, pronouns, and adjectives lost almost all of their case