Arabic internal plurals of nouns and adjectives (also known as "broken" plurals) raise a challenge for morphological theory in that they do not obviously involve affixation of a plural morpheme analogous to, e.g., English /-ez/. Rather they show a contrast of forms that share the same consonantal root, but differ in vocalization and possibly on other counts as well. This article proposes an analysis of internal plural formation that derives all forms through a single process, viz. inserting one of the three glides /I/, /U/, and /A/ into a specific site located between the 2nd and 3rd root consonants, the post-C2 site, thereby creating an onset (C) or a nucleus (V) position. It then explores the consequences of this account for the overall organization of grammar. A modified version of Distributed Morphology is put forward that construes the lexicon as a set of sets of elements endowed with meanings but no form, and where Spell Out is a function that takes element sets and returns legitimate phonological objects, i.e. CV strings.