Analysis of information sources in references of the Wikipedia article "Spurious diphthong" in English language version.
The sub-dialects of Laconia, Crete, and Southern Italy, and of their several colonies, are often called Severer (or Old) Doric; the others are called Milder (or New) Doric. Severer Doric has η and ω where Milder Doric has ει and ου.