# Rule of product

(Redirected from Fundamental Counting Principle)
The elements of the set {A, B} can combine with the elements of the set {1, 2, 3} in six different ways.

In combinatorics, the rule of product or multiplication principle is a basic counting principle (a.k.a. the fundamental principle of counting). Stated simply, it is the idea that if there are a ways of doing something and b ways of doing another thing, then there are a · b ways of performing both actions.[1][2]

## Examples

${\displaystyle {\begin{matrix}&\underbrace {\left\{A,B,C\right\}} &&\underbrace {\left\{X,Y\right\}} \\\mathrm {To} \ \mathrm {choose} \ \mathrm {one} \ \mathrm {of} &\mathrm {these} &\mathrm {AND} \ \mathrm {one} \ \mathrm {of} &\mathrm {these} \end{matrix}}}$

${\displaystyle {\begin{matrix}\mathrm {is} \ \mathrm {to} \ \mathrm {choose} \ \mathrm {one} \ \mathrm {of} &\mathrm {these} .\\&\overbrace {\left\{AX,AY,BX,BY,CX,CY\right\}} \end{matrix}}}$

In this example, the rule says: multiply 3 by 2, getting 6.

The sets {A, B, C} and {X, Y} in this example are disjoint sets, but that is not necessary. The number of ways to choose a member of {A, B, C}, and then to do so again, in effect choosing an ordered pair each of whose components are in {A, B, C}, is 3 × 3 = 9.

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.

Other typical example is using it with the rule of sum, in this case we have two groups, the group A with 3 elements and the group B with 10 elements. We want to pick one element ( we don't care if it is from group A or B) and a second element that must be from the group B. The way that we can chose the elements are:

${\displaystyle \mathrm {Total} \ \mathrm {ways} =(3*10)+(10*9)}$

First, we use the rule of product to get the number of ways if we pick a element from group A and then from group B. After this, we repeat the process but now changing the element of group A by element of group B and multiply by the number of element in B - 1 because we pick one of this.

## Applications

In set theory, this multiplication principle is often taken to be the definition of the product of cardinal numbers.[1] We have

${\displaystyle |S_{1}|\cdot |S_{2}|\cdots |S_{n}|=|S_{1}\times S_{2}\times \cdots \times S_{n}|}$

where ${\displaystyle \times }$ is the Cartesian product operator. These sets need not be finite, nor is it necessary to have only finitely many factors in the product; see cardinal number.

## Related concepts

The rule of sum is another basic counting principle. Stated simply, it is the idea that if we have a ways of doing something and b ways of doing another thing and we can not do both at the same time, then there are a + b ways to choose one of the actions.[3]