  Chomsky argued (on technical grounds; see Chomsky 1957, ch.1) that  grammars must be enriched with a second type of rule, known as  ‘transformations.’ Unlike phrase structure rules,  transformations operate on whole sentences (or more strictly, their  phrasemarkers); they allow for the generation of new sentences  (/phrasemarkers) out of old ones. The Passive transformation described in Chomsky 1957:112, for instance, specifies how to turn an active sentence (/phrasemarker) into a passive one. Simplifying  somewhat, you take an active phrasemarker of the form NP —  Aux — V — NP, like Kate is biting  Mark, and rearrange its elements x1 —  x2 — x3 —  x4 as follows: x4 —  x2 + be + en —  x3 + by — x1  to get Mark bite (+ is + en) by  Kate. The parenthetical + en and + is invoke  further operations on the verb bite that transform it into  is being bitten, and ultimately Kate is biting Mark is ‘transformed’ into Mark is being bitten by  Kate.