Ten cards, one decision.

Melds are three or more of a rank, or three or more in sequence in one suit. Everything left over is deadwood — aces one, pictures ten, the rest face value. Knock at ten deadwood or less; knock at zero and it is gin, worth twenty-five extra. Knock badly and your opponent can lay their deadwood onto your melds and undercut you, which is worth twenty-five to them.