Why the 5′s are the Most Important Cards

If you succeed at counting cards in blackjack, you will certainly raise you chances of winning. Generally, the rule of thumb here is when the deck has many 10′s, it is seen as better for the player for the reason that the dealer will generally bust. Because of this, you, the player, will hit more often.

Mathematically, the 5 card can be the most interesting. Back in the day, there were systems for card counting that involved only the 5 card. Once it was seen that the 5′s were gone, it gave players an advantage and that was the time to raise the bets.
Each time a 5 came out of the deck, it could add 0.67% better odds for the players return. This would of course only be feasible in a one deck game. This translates into, just one 5 card gone from the deck will create an small but meaningful advantage over the house. And gaining a better edge over the casino would be two of three 5′s out of the deck. This is a rare situation for sure but if you know about it, you can use it when the time comes around.