until the advent of Islām (7th century AD); the Zoroastrian sacred book, the Avesta, is written in a variant of Pahlavi called Avestan.

The Pahlavi alphabet developed from the Aramaic alphabet and occurred in at least three local varieties: northwestern, called Pahlavik, or Arsacid; southwestern, called Parsik, or Sāsānian; and eastern. All were written from right to left. Of the 22 letters in Aramaic, most came to represent more than one sound in Pahlavi; several were not used at all, and one evolved into two letters in Pahlavi. Northwestern Pahlavi had 20 letters, and southwestern had 19. Avestan, a cursive script, had 50 distinct letters and was perhaps separately invented, though patterned after Pahlavi.

A peculiarity of the Pahlavi writing system was the custom of using Aramaic words to represent Pahlavi words; these served, so to speak, as ideograms. An example is the word for “king,” in Pahlavi shāh, which was consistently written m-l-k after the Aramaic word for “king,” malka, but read as shāh. A great many such ideograms were in standard use, including all pronouns and conjunctions and many nouns and verbs, making Pahlavi quite difficult to read.