Basic blackjack strategy says to hit if you think that you are currently holding a hand lower than the dealer’s hand. However, this is constrained by the caveat that the dealer must hit below 17 and stand at 17 and above. Therefore, it can be wise for a player to stand with a 15 if the dealer showing perhaps a 5 as the dealer must hit and the odds are in favour of busting. In general, the odds are that the dealer’s face down card is valued at 10 and a beginner would be wise to assume this.
A beginning player would be wise to stand with any hand above 17 as the dealer is required to stand at 17 and there is a high probability that hitting in this position will result in a bust.
A player should double-down on a hand of 10 or 11 if the dealer’s face-up card is anything other than a card valued at 10 or an Ace; based on the assumption that the dealer’s face-down card is a 10 and the odds are in favour of the player receiving a 10 as well.
More advanced strategies involve a variety of methods of trying to calculate the likelihood of what card will be dealt next and involve different card counting strategies. Specific strategy will vary due to the rules of the specific game. The number of decks involved changes the odds of certain cards being turned up. As well as the rules regarding doubling-down on split hands and rules that allow for re-splitting change the options a player can pursue as well as the odds of success against the house.