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Key cards, blockers, and the second-nuts trap

Knowing "a flush is possible" isn't enough. The skill is knowing which two cards make it, because then you can ask the only useful follow-up: do I hold one of them?

That card is the key card. On a three-heart board the key card for the nut flush is the highest heart not on the board. On the Q-J-9 board above, the key card is any ten.

This pays off in two directions:

Defensively. If you hold a key card, the nuts is less available than the board suggests — see ♥R, this is exactly what a blocker is.

Offensively. If you can't have the nuts and your opponent's betting says he does, believe him. Which brings up the classic beginner's grave:

The second nuts is a trap. The second-best flush, the second-best full house, the low end of a straight — these hands feel enormous and are the most expensive holdings in poker, because the only hand that beats you is exactly the hand people bet big with. When you hold the second nuts and face a large bet on the river, you're usually calling to lose.