Reef Five

Five cards each

You and the host both receive five cards. Yours are face up, the host's are hidden until you have acted.

Look, then choose

Either fold and lose the opening, or double it to see the showdown. There is no drawing and no third option.

The host's threshold

The host only qualifies with an ace and a king or better. If the hand does not qualify, the opening pays and the doubled part is returned untouched.

A single threshold rule

Play everything ace-king-high or better and fold the rest, and you are playing this game correctly. The whole strategy fits in one sentence.

Why the margin is wide

At 5.22 percent this is one of the wider card games anywhere, and the reason is the qualification rule: when the host does not qualify, your doubled stake earns nothing at all.

Why cards are different

A grid draws afresh every round. A deck does not: every card that has already appeared changes what is left, so the odds move while the hand is in progress. That is the only family here where what you have seen tells you something.

Order of play is the whole game

In most card games the house acts last and knows more when it acts. The rules that look arbitrary — draw to sixteen, fold below a threshold, act before the fifth card — are all attempts to price that asymmetry.

Where the margin lives

A well-played card game has one of the narrowest margins anywhere: a fraction of a percent. Played carelessly, the same game can cost ten times that. The gap between the two numbers is the value of knowing the rule.

A caution about systems

Doubling after a loss does not change any game's margin. It changes the shape of the loss — many small wins and one very large loss — and the total expected result stays exactly where it was.