ass-tag convention:
The accepted standard that the tagged side of a towel is for designated nether-regions while the non-tagged side is reserved for the face and hair. This convention is used to avoid cross-contamination and is to be assumed as general practice. Practical in situations where one must use a borrowed or foreign towel.
"I forgot my towel in the shower and had to use my roommates. Hopefully he observes the ass-tag convention."