I've been debating this for a while now with a comrade of mine. Wikipedia (and others) give "close helmet" as a type of medieval helmet.
My compatriot argues that, as-is, this doesn't make sense, that it should be either "closed helmet" (with an extra D) or "close-helmet" (hyphenated). I say "close helmet" is perfectly fine, and simple Google searches turn up more results for this form of it, but I can't find a convincing argument for why it is correct other than because it seems to be the most common usage.
My friend says that "close helmet" is only correct if "close" is an adjective, as in "she sat close to me", but that doesn't really make sense for a type of helmet, since ALL helmets can be considered "close" to one when worn.
Yet, using "close" as a verb ("I shall close the door") doesn't really make grammatical sense either. The defining feature of this type of helmet is that it can open and close (or it is related to the opening/closing; I'm not entirely sure of the details), so "close helmet" makes some kind of sense, but I don't know a grammatical construction that allows this.
Or is this just a relic of older English that has survived into the present?
At first, I was sure that "close helmet" was correct, but the more I think about it the more confused I make myself. Help?