Who can buy letters?

[sundance]

This is probably a dumb question...

Is there any restriction as to who can buy letters?

My little sister (blood relation, not sorority!) bought me a sweatshirt with my letters on it. I love it, but I just wanted to make sure that she didn't break some rule about having to be a member to buy letters, and that I'm not breaking any rules about wearing letters that aren't self-purchased or from my big.

In that case, would I be able to buy another sorority's letters for a friend who was in that sorority?

Thanks