Yes, it's true. Clover circular identifies US 6s as 4.25 mm, while Skacel Addi and Bryspun identify US 6s as 4.0 mm.
Did you know it's safer to go with the mm size on Addi Turbos because their US sizing can be off?
Yes, it's true too.
How did you learn that real knitters use the metric system? Please share.
Impetus for learning, and above link, courtesy of email exchange with Wendy B. regarding my Fad-Classic swatch.
I hope she doesn't think I'm a total neophyte.