Also known as the two urinal rule. In a mens' bathroom with a row of urinals, whenever someone is using a urinal, the next man to come in and use a urinal will go at least 2 stalls down (ie, you are at stall zero, the one beside you is stall one, the next one is stall two). Whenever someone violates the two stall rule, the person who is violated feels uncomfortable, even if it is the violator's best friend.
Sinks count as stalls as well.
Man, he just violated the two stall rule by pissing next to me!