Everyone knows how to play the game, and most probably know how to play many system of blackjack . 
Napoleon was a big fan of the game. During his exile in Elba, then later in St Helena, he used to play blackjack frequently.
The earliest versions of the game are about 500 years old. It was invented by either the French.
Blackjack got its name in the early 1900s. At the time, the player got a bonus if an ace of spades was accompanied by a jack of either clubs or spades, before then, the game was just called 21.
Statistically, you are more likely to lose when you double down. But when you do win, you win twice as much, so in the long run, doubling down is good.