Casino rules for the game blackjack vary from casino to casino, from pit to pit and even from table to table. For example, on a single-deck game the casino might allow doubling on 10 and 11 only, while at the very next table with a shoe game they might allow doubling on any two cards. These different rules are know as house rules. You should check the placard at the table to be sure you know what the rules are for that table, pit and casino. Only pay at those casinos that offer the best house rules for the player. Remember, the better the rules, the better your win-rate will be.