Think of blackjack as a kind of gambling with statistics. You're essentially putting your own math skills up against the casino's. The casinos have figured out what rules are most likely to put your money in their pocket and they've established tables using those rules. To keep the game interesting, the dealer is operating without free will. Blackjack players aren't held to the same code--you're totally free to hit on 18 or stand on a pair of 4s should you so desire. The dealer, on the other hand, has to play by certain rules, standing at a certain total and hitting up to that total, etc.
Combine these rigid rules of behavior for the dealer with a basic understanding of statistics and you can be a real danger to any blackjack table in any casino. Winning money at blackjack requires you to take informed chances when the advantage is in your favor, meaning when you have a high probability of beating the dealer. Remember--the object of blackjack is not to get as close to 21 as possible, it's to get a higher point total than the dealer or to wait out a dealer bust.
You can make blackjack strategy really complicated, or you can look at the probabilities of different hands and apply simple blackjack strategy. There are a couple of facts about the game of blackjack that should inform your game decisions.
In a given deck of 52 cards, the odds of drawing any 5 are 4-out-of-52. Do the math and you find that works out to 7.7%. The odds of drawing a 6 or a 9 are also 4-out-of-52. The odds of drawing any one numbered card are always 4-out-of-52, except for cards with a point total of 10. Cards valued at 10 points are different because there are more of them than any other card. Numbered tens, jacks, queens, and kings are all worth 10 points. That means there are sixteen cards with a value of 10 points, so the odds of drawing a 10 point card are 16-out-of-52, which is 30.8%. You are four times more likely to draw a card worth ten points than any other card. Since 10 point cards are so common, blackjack strategy pays special attention to the probability of you or the dealer drawing a 10.
Using 10s to Your Advantage
The high likelihood of a 10 point card popping up is the main reason why a 5 or a 6 up card for the dealer can give the player such an advantage. It is also the reason why a player point total of 12 or more can be a really tricky play.
If you read into blackjack probability and theory, you'll hear the phrase "bust probability" from time to time. This is a number representing how often a certain point total will bust. Obviously, lower point totals have a lower likelihood of busting. It is impossible to bust if your point total is 4, but highly likely to bust if you draw against a point total of 19.
Dealer Bust Probabilities
Dealer bust probability is more complex, considering that the dealer must stand at 17. The dealer is much more likely to bust with a point total of 16, because he is forced to take a hit. In fact, on any point total up to and including 16, the dealer must hit. Once the dealer hits 17 or more, he stands. The blackjack rules give him no choice, and that's another way in which the player has an advantage. This requirement to hit is why dealers with low point totals after the deal are more likely to busy--they have to draw up to 17, and a dealer whose up card is a 5 or a 6 is very likely to draw a 10, bringing his point total to 15 or 16. At that point, he is forced by the rules of blackjack to draw again, with the likelihood of drawing a card higher than 6 or 7 being pretty high.
Using blackjack probabilities to your advantage is called "basic strategy," and there are plenty of places to find blackjack strategy charts that do the hard work for you. Coming to grips with the power of the many 10-value cards in blackjack is the first important step towards improving your blackjack game. Learning to manipulate the dealer's lack of free will is the best way to learn how to use blackjack probabilities to your advantage.