A link—mathematical or not—that describes a relationship between the elements of two sets. In many cases, one of these sets is an ordered sequence of numbers called ranks.

A rule is also called a mathematical

*law*or*condition*.### Examples

- The rule of a function is the expression that defines the relationship that exists between each input value (domain) of a function and the output value (range).
- An exponential rule is an expression that defines a relationship based on the exponentiation operation.
- A linear rule of the y = mx form is an expression that defines a relationship of proportionality.
- A logarithm rule is an expression that defines a relationship based on the concept of logarithm.