Darmok wears a 16" -18" in better quality collars, or a 16" - 20" in the Wal-Mart variety. Sasha wears a 14" - 16" because she has that beautiful thin, long arched neck. Dar's is more short and thick. I bought a really great collar at Petsmart over the weekend, though...it lights up with blinking red & green christmas lights all along the neck, and it's a heave triple-thick webbing with a heavy brass buckle closure. I love it!! I walk Dar every morning before I go to work, and it's just as dark at 6 am as it is at midnight, so it's not only decorative and festive (haha) but it serves a purpose as well---I can see where she is! It's hard to see a black dog in the black night!