But ... when you have one somewhat "loud" pattern (say, a loud tie or a loud shirt), if you surround it with all-plain this and that, you magnify the power of the loud pattern. If one posits that clothes should blend together in such a way so that no one item stands out from the rest, it might well be that a shirt with, say, large checks should be paired with a large-patterned tie so that neither takes center stage. The patterns cancel out and all is good. It might not be everyone's taste to wear combinations like that, but I think the bias against multiple patterns can sometimes be counterproductive. It invites the famous scene from Animal House when the frat brothers descended upon some poor rush with "Hey - nice tie! Boomer (or whatever), come over here and check out this tie!"