rule of three

Alternative forms

 * the Rule of Three

Proper noun

 * 1)  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)  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)  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)  Specifically, the technique of having a setup in advance, a reminder to keep it fresh in the audience's memory, and then a payoff.
 * 5)  A religious tenet stating that the energy a person puts out into the world, positive or negative, will be returned to that person threefold.
 * 6)  A rule of thumb in the C++ programming language, stating that any class that defines a destructor, a copy constructor, or a copy assignment operator should define all three of them.

Translations

 * Finnish: kolmen sääntö
 * French:
 * Polish: reguła trzech
 * Romanian:
 * Spanish: regla de tres