What size is a 12 mm crochet hook?
The crochet hook size 10 mm refers to the millimetre measurement. It is also known as the US size O.
What letter size is a 12 mm crochet hook?
It is a US size O crochet hook.
What size is a O crochet hook?
A size O crochet hook is the same as a size 12 mm crochet hook. Don't mix it with a size 0 (zero) crochet hook.