Modern Farsi historically possessed an underlying opposition of vowel length that has since largely disappeared in all but open, non-final syllables. At the same time, qualitative differences have arisen between the once identical short and long vowels. Two opposing analyses have been proposed to account for these facts. The “quantity only” analysis posits an underlying opposition of length and derives the qualitative differences by rule, while the “quality only” analysis removes length from the underlying representation but includes the differences in quality. In this paper, it is argued that neither analysis is able to describe the vowel system both concretely and with maximal generalisation. A synthetic analysis is proposed that integrates quantity and quality in the underlying vowel system and is able to account adequately for the observed phenomena. It moreover sheds light on the modern Farsi vowel system as a transition state between the historical “quantity only” system and the system of a future Farsi possessing an opposition of quality alone.