Really depends on my mood and destination. The only rule I have been following lately is to wear a tartan on Mondays. Why you ask? Because Mondays are new hire day, and all of the people joining the company are sitting in the lobby of the entrance I use to get to my office. So I get to greet every new hire (well, they get to see me on the way in), and I figure a tartan is easier for some people to identify. However, 2 weeks ago when Monday morning was in the single digits, I wore the lined PK and did not worry about it.