Tell (blackjack)
A dealer's give-away expression, gesture, mannerism or overall
attitude which gives to the alert player enough information
about the dealer's hole card. Tells exist only in games where
the dealer checks under tens for a natural. To guard against
tells, and also dealer-player collusion, most casinos have installed
automatic hole-card checking machines or switched to the no-peeking-under-tens
rule.
