Abstract Arabic dialects west of Libya stand out within Arabic and Semitic languages more generally for using a fairly productive verb pattern FʕāL for change-of-state quality verbs, since before the 12th century. This has long been identified with Classical Arabic’s Form IX (iFʕaLLa) or XI (iFʕāLLa), but does not regularly correspond to either, and shows a rather different lexical distribution. This article proposes a new explanation for its unexpected form, greater productivity, and uniquely Western distribution, based on contact with Berber. Across most Berber varieties, a verb pattern whose perfective stem may be reconstructed as *ăFSāL > *əFSāL is regularly used for change-of-state verbs. The originally accidental similarity in form and function would have allowed first-language Berber speakers learning Arabic in the Umayyad period to identify this with Arabic Form IX/XI, and therefore to extend its use to their Arabic.