In Ancient Greek, how common is this declension? It’s in the second declension group but called “attic declension.”

To add to the others:

• Vote #1: Humphry Smith’s answer to In Ancient Greek, how common is this declension? It’s in the second declension group but called “attic declension.”
• Vote #1: Robert Todd’s answer to In Ancient Greek, how common is this declension? It’s in the second declension group but called “attic declension.”

The Attic declension is indeed specific to Attic: it represents a sound change specific to that dialect, whereby V̄ο > V̆ω, where V is any vowel that can be long. So Doric νᾱός, Ionic νηός, Attic νεώς.

As Robert Todd said:

Personally, I didn’t memorize these. I pick them up in continuous reading and apply the following mental adjustment to the “ο” stem second declension specimens – it it’s ο, ου, α then ω, if οι then ώ.

With λαγώς, you’re also seeing some vowels being merged together.

This is an annoying peculiarity of Attic, and Koine dropped it like a stone; abandoning the Attic declension is in fact a major source of Doric words in Koine.

I don’t even go as far as Robert in my memorisation: I just think “Oh, an omega is there. Attic Declension. I’ll treat it like an omicron. Or an omicron upsilon. Whatever works.” They really are just second declension nouns with a long final vowel.