(chess) The rule stating that, if a player intentionally touches a piece when it is their turn, they must move or capture the piece if it is legal to do so.

(chess) A rule in professional chess that a player may declare the game to be a draw after 50 consecutive moves by each side without any capture or pawn movement, usually invoked during an inconclusive endgame.

(law) The presumption that the failure of a party to produce a witness available to them indicates that the testimony of that witness would go against the party.

The rare occurrence of a counterexample to a rule, used to demonstrate that the rule exists.

(idiomatic) A clear-cut, easy to make a decision. / (law) A clearly defined rule or standard, composed of objective factors, which leaves little or no room for varying interpretation.

Any rule that prescribes or proscribes behavior during a ten-second interval, especially:

Any rule that prescribes or proscribes behavior in a three-second interval, especially

Alternative form of the hand that rocks the cradle rules the world

(computing, informal) The rule that a database, a user interface, etc. should allow for either zero, one, or an indefinite number of an entity.

(statistics mnemonic) The rule stating that a normal distribution will have 68% of its observations within one standard deviation of the mean, 95% within two, and 99.7% within three.