(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 a/b=c/d then d=(bc)/a.

The regulation requiring all traffic (travelling on a road or otherwise) to keep either to the left or the right. / (nautical) A regulation concerning safe handling of vessels on seaways.

(Internet slang, fandom slang, informal) The proposition that it is possible to find genderswapped versions of every fictional character, especially as fan art on the internet.

A doctrine in antitrust law which applies to contracts which "unreasonably" inhibit trade.

(law) A notion of what should "count" as law.

(photography, film, visual arts) A normative rule that an image should be mentally or representationally divided in three horizontally and diagonally, and that the focus of the image should not be in the center of the resulting shape.

plural of slide rule

plural of formation rule

(Ireland) international rules football

(logic) Any of a set of rules used to construct well-formed formulas of a formal language.