It's simple. Rules that everyone SHOULD know, but sometimes don't.
It'll be interesting to see what some cultures on this site consider unwritten rules!
1. No man, regardless of physical condition or aesthetic value, should wear a speedo at a public swimming pool.
2. The length of any man's shorts, should not be shorter than one inch above the knee.
3. Mullets are frowned upon, wear them with caution, preferably not at all.