If you walk into a bathroom with say, 5 urinals, and the first and last are in use (as they should be in a case where 2 dudes are pissing in a 5 urinal bathroom), then you must take the middle urinal. If you are the fourth dude to walk in and it's like this - dude pissing, empty, dude pissing, empty, dude pissing - then you must take urinal #2.