The key card concept applies to single deck only. The idea is that key card information often have a greater importance than the count itself. For example, everyone knows that once all the aces come out a blackjack is impossible. But what if all the 8's were played and you receive a 13 versus a dealer 10. Let's say the count is around 0. What do you do? The answer is you stand on this hand -- not hit! That is what the key card concept is all about. If you have a 14 your key card is the 7, etc., etc.

I don't know of anyone that has published a complete set of play variations based on key cards. If someone has done this work I would love to publish it in Blackjack Review.