When we were but impressionable youth, we did gather around the TELEVISION and did watch the soon-to-be-canceled show Undeclared, on FOX.
The first ruled agreed upon by the main character and his Casanovian roommate was that to tie a blue scrunchie securely to the outer doorknob of their shared room was the signal that shared space was being used by the tie-er an a significant other.
This rule was forever burned into our impressionable teenage brains, and upon becoming roommates we (Dr Subtle and Monseigneur Rorschach) immediately agreed upon it. Having had previous roommates our definitions had both loosened- the "blue scrunchie" had been generalized to "handy cloth thing"- socks, belts, ties, pants, tshirts. This gesture was not a new one- it was a lazy copy of a stolen one. Until Monseigneur Rorschach innovated not a week back.
Petite Amie's Top
Upon returning home one day Dr Subtle found not a gym sock or old tie around his and his roommate's doorknob, but indeed, did find instead a frilly woman's top. Rorschach's Petite Amie's top. On the outside of the door. The locked door. Ahh, the doctor thought. A practical man my roommate is.
Upon retiring some days later with his own paramour, the Doctor did borrow her top, and did exchange the favor. This significant gesture continues to see use.