(chemistry) A set of three rules that invoke the conservation of orbital symmetry to predict the stereochemistry of pericyclic reactions.

According to an urban legend, an unofficial guideline used by publishers and the media in the United Kingdom to decide to what degree of erection an image of a penis can be shown.

(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.

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

(sports) A rule whereby, in case of an aggregate draw, the team with the most away goals wins.

(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.

(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.

plural of prudent man rule

A common mnemonic for understanding notation conventions for vectors in three dimensions. / (logic) The systematic use of right-hand turnings to solve a maze.

(chiefly Wikimedia jargon) It is acceptable to break rules in situations where it is beneficial not to follow them.