The treatment of Proto-Iranian *θw (Proto-Indo-European *tw) is one of the isoglosses distinguishing Middle Persian from Parthian and thus important for Western Iranian dialectology. The re-discussion of the Parthian development of this consonant cluster by Nicholas Sims-Williams presents a welcome opportunity for some notes on the matter. I will argue that there is some additional evidence in favour of his suggestion that the Parthian result is not -f- as previously assumed, but a consonant cluster. I will also suggest a modification of the steps that the development takes. The Middle Persian development of *θw as well as some related issues of historical phonology and Parthian orthography and Western Iranian sound changes are likewise discussed.