Rabbenu Tam, Where are you when I need you?
We're keeping our house cold to lower our heating bills. I found the cold unbearable until I started wearing thin, black “duofold” underwear designed to keep me warm. I call this blessed stuff “middlewear”, since it goes between my shirt and undershirt.
Every morning, I have to decide whether to place my tallit katan under, or over, my middlewear. I think this is a matter of taste that has no practical consequence, so I don't always make the same decision. That may explain why, this morning, I put the tallit katan under my middlewear. And I put the other tallit katan over my middlewear. My intention, of course, is NEVER TO DO THAT AGAIN!