When something always does the same thing, one can say that there is a rule that it does what it does.
The word "rule" has many uses, including the following:
- A means of measurement (such as a slide rule.)
- A law or regulation.
- A precedent or conclusion drawn from experiments.
A "rule" in mathematics is a formula which allows a person to work out parts of the solution if he or she has certain information. For example, Pythagoras' theorem is a rule which states that if a person knows the lengths of two sides of a right-angled triangle he or she can work out the length of the third.
Rules, however, can also means laws created by people in power - for example, school rules are the "laws" of a specific school.