Every card has four features: number (1–3), shape, colour and shading. Three cards make a set when each feature is either the same on all three or different on all three.

• Three greens, all diamonds, all striped, with 1 / 2 / 3 symbols — a set.
• Two solid and one striped — not a set.
• No set here deals three more cards; if you are wrong it costs 10 seconds.
• Hint lights up a card that is part of a set (+15 seconds).

Any two cards have exactly one card that completes their set — the game is a small exercise in finite geometry, and a fierce pattern-recognition workout.