Back to the non-rule rules:
1. If I wear a suit without a tie, I wear a patterned shirt. It looks less like I just forgot to put on a tie. Still, I do this as rarely as possible.
2. We are "business casual" at my office, but I wear a jacket or blazer or suit coat every day. No casual Fridays allowed.
3. I have casual cotton jackets that I wear in the summertime, but I don't pair them with nice wool trousers. Cotton goes with cotton. I have been known to wear a navy linen...