Unless disciplined, a dog becomes a household pest.
Is the use of unless in this sentence correct? It sounds to my naive ear to be fine, but I always think of unless as a conjunction, which does not seem to be the case here. I looked up unless in an online dictionary and discovered that it can additionally be used as a preposition in the sense of "except for", but again that would not seem to apply to this sentence as disciplined is an adjective. Since the sentence is taken from Strunk and White and sounds right to my ear, I am sure it is correct, so my main interest is to discover what the underlying construction is here.