# rule of three

## English

### Proper noun

the rule of three

1. (mathematics education) A rule stating that, if two ratios are equal, then the denominator of the second equals its numerator times the reciprocal of the first ratio: if $\frac ab=\frac cd$ then $d=\frac{bc}a$.
2. (medicine) A rule for clinical trials used for determining the rate of adverse side effects when no such side effects present during the course of the trial.
3. (writing, rhetoric) A rule which states that things which come in groups of three are inherently funnier or more effective than things which come in groups of other sizes.
4. (religion, Wicca) A religious tenet stating that the energy a person puts out into the world, positive or negative, will be returned to that person three times.

#### Quotations

For usage examples of this term, see the citations page.