What is the "Rule of 6"?

I recently got this question via email: “What is the rule of 6?”

Casinos that deal single-deck blackjack often instruct their dealers to use the “rule of 6″ to decide when to shuffle. This means that the number of players plus the number of rounds between shuffles should add up to 6. If there are 5 or more players at the table, you’ll get one round and then a shuffle. (Ick.) Four players get two rounds, Three players get three rounds, Two players get four rounds, and a single player heads-up will get 6 hands between shuffles.

So, now you know.