Rule of product

As another example, when you decide to order pizza, you must first choose the type of crust: thin or deep dish (2 choices).

Next, you choose one topping: cheese, pepperoni, or sausage (3 choices).

Using the rule of product, you know that there are 2 × 3 = 6 possible combinations of ordering a pizza.

In set theory, this multiplication principle is often taken to be the definition of the product of cardinal numbers.

The rule of sum is another basic counting principle.