Counting cards in blackjack is a way to increase your chances of winning. If you're good at it, you can actually take the odds and put them in your favor. This works because card counters raise their bets when a deck rich in cards that are advantageous to the player comes around. As a general rule of thumb, a deck rich in 10's is better for the player, because the dealer will bust more often, and the player will hit a blackjack more often.

Most card counters keep track of the ratio of high cards, or 10's, by counting them as a +1 or a -1, and then gives the opposite +1 or -1 to the low cards in the deck. Some systems use a balanced count where the number of low cards is the same as the number of 10's.

But the most interesting card to me, mathematically, is the 5. There were card counting systems back in the day that involved doing nothing more than counting the number of fives that had left the deck, and when the 5's were gone, the player had a big advantage and would raise his bets.

A good basic strategy player is getting a 99.5% payback percentage from the casino. Every 5 that's come out of the deck adds 0.67% to the player's expected return. (In a single deck game, anyway.) That means that, all other things being equal, having one 5 gone from the deck gives a player a small advantage over the house.

Having two or three 5's gone from the deck will actually give the player a pretty significant edge over the casino, and this is when a card counter will usually raise his bet. The problem with counting 5's and nothing else is that a deck low in 5's happens pretty rarely, so gaining a big advantage and making a profit from that situation only comes on rare occasions.

Any card between 2 and 8 that comes out of the deck increases the player's expectation. And all 9's. 10's, and aces increase the casino's expectation. But 8's and 9's have very small effects on the outcome. (An 8 only adds 0.01% to the player's expectation, so it's generally not even counted. A 9 only has 0.15% affect in the other direction, so it's not counted either.)

Understanding the effects the low and high cards have on your expected return on a bet is the first step in learning to count cards and play blackjack as a winner.