blackjack cheat card

Is "count" cheating when playing cards blackjack casinos?
I'm not sure if "counting" the cards, as in the film 21 is cheating. Especially in the casinos. But I think in the movie, were arrested for being in a team .. so they have more money this way. I'm not sure. Do not count cards when playing blackjack or not cheating in casinos?
More no doubt counting cards makes the game beatable blackjack only in the … through 6 or 8 decks with 5-6 players at the table.
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