 However, it is possible that in reality, final comes from "long" *"-ū" < "-um", where original final "-m" caused vowel lengthening as well as nasalization.Evidence of this comes from Rhaeto-Romance, in particular Sursilvan, which preserves reflexes of both final "-us" and "-um", and where the latter, but not the former, triggers metaphony.This suggests the development "-us" > > , but "-um" > > .