The counting strategy which I'm going to talking about here is called Hi/Lo, but there are others.
The Hi\Lo is famous because it was used by M.I.T Blackjack Team which, along with teamplay gained millions.
This counting strategy separates cards in three groups: (2, 3, 4, 5, 6,), (10, J, Q, K, A) e (7, 8, 9). The first group (2, 3, 4, 5, 6,), are good when removed from the deck, so they get a positive count, +1. The second group (10, J, Q, K, A), are bad when removed so they get a negative count -1. The last group (7, 8, 9) is considered neutral so they are worth 0. Higher your counting higher your advantage over the dealer. There are other two concepts: True count: which is to divide the running count by the number of remaining decks. And Optimum bet: which is the optimization of your bet in based in the count.
