Dr. Thorp developed the first method of card counting. This method is based on counting cards with a value of 10. This means that the counter is actually determining the percentages of 10's against the other cards in the deck. This is used to determine if the deck is favorable to the player or the House.
There are 16 10 value cards and 36 non 10 value cards in a deck of cards. At the beginning of a game the deck stands at 2.25 to 1 which is slightly in favour of the house. When the percentage drops below 2.25 to one the deck is then in favour of the player. When the deck favours the player they increase
their bet and conversly when the deck favours the house the player lowers their bet.