I believe that the OP honestly doesn't know what a T-shirt is and needs help with this.
OP..."You hate your job? There's a support group for that. It's called EVERYBODY and they meet at the bar." The quote is from Drew Carey ("Mr. Parma" and the star of "Sounds Dangerous", among other things). You'll find that you're much happier at work when you focus on the positives and not the negatives.
Good luck with figuring out what is and isn't a T-shirt.