Set is a real-time card game. The deck consists of 81 unique cards that vary in four features across three possibilities for each kind of feature: number of shapes, shape, shading, and color. Each possible combination of features appears as a card precisely once in the deck. In the game, certain combinations of three cards are said to make up a set. For each one of the four categories of features — color, number, shape, and shading — the three cards must display that feature as a) either all the same, or b) all different.