OXYTONE • FREE TRANSLATOR.ORG • Definition of Oxytone

... and this contracted would become Dyaus. Thus we have paribhv from paribhs. In Greek the facts are the same, but the explanation is more difficult. The general rule in Greek is that vocatives in ou, oi, and eu, from oxytone or perispome nominatives, are perispome; as plako, bo, Lto, Ple, basile, from plakos, ontos, placenta, bos, Lt, Ples, basiles. The rationale of that rule has never been explained, as far as Greek is concerned. Under this rule the vocative of Zes ...

— Chips from a German Workshop - Volume IV - Essays chiefly on the Science of Language • Max Muller