We know what this card is. You see it all of the time when playing blackjack. But you might not think that this particular card packs the punch that it does in terms of blackjack strategy. Or maybe you do and you just do not realize it.
There is one card in all of blackjack, be it casino blackjack, online blackjack, Spanish 21, Pontoon, etc, that all blackjack strategy is based on. Players act upon this card and they may not even be fully conscious that they are doing so. To them they are just going with their strategy.
But strategy has to be based on something even blackjack strategy.
In the case of blackjack strategy the most important card, the card that all of your strategy is based on is the dealer