A couple of weekends ago I was chatting with a knitter friend, and she commented that her 16" needles "felt short" to her. She guessed they were closer to 15 1/2". We measured them, and she was spot on!
I don't know how on earth she could tell...
Peg and Salsa, that's interesting trivia about the 40cm measurement for Addi's. I did measure my 16" addi's, after that encounter with my friend, and I've been curious to know why they weren't exactly 16". That makes total sense! ...Not that I was bothered by it. Better to have them a bit small, than too big for your project!