I've been trying to look for quotes on retractable blades, specifically the kind where one is worn on the wrist. I think Inigo Skimmer had one in the Fifth Elephant, though not sure. I know someone was seen with it (velvet on one side). But also I remember some line about them being a fool's weapon, or banned by the assassin's or something like that in a different book. I'm trying to remember the first reference to them.
"The reason an author needs to know the rules of grammar isn't so he or she never breaks them, but so the author knows how to break them."