The Projection Principle is a stipulation proposed by Noam Chomsky as part of the Phrase Structure Component of Generative-Transformational Grammar. Under the Projection Principle, the properties of lexical items must be preserved while generating the phrase structure of a sentence. The Principle, as formulated by Chomsky in Knowledge of Language: Its Nature, Origin and Use, states that "lexical structure must be represented categorially at every syntactic level". For example, the verb strangle, apart from the subject, has an obligatory argument, its object, which must appear in the sentence. The following subcategorization frame for the verb strangle specifies its properties; the underlined gap for the location of the verb is followed by the noun phrase (NP): Noam Chomsky; photo credit: John Soares Avram Noam Chomsky (b. ... It has been suggested that this article or section be merged with Generative linguistics. ...

strangle Verb, [__ NP]

It is out of this frame that a sentence like the following can be generated:

Fabio strangled Prince Jamal.

A sentence without the object, in violation of the verb's subcategorization frame and the Projection Principle, would be ill-formed:

^*Fabio strangled.

Before the Projection Principle was proposed, phrase structures were generated in separation from the properties of lexical entries. These were hypothesized to enter the slots in pre-generated structures waiting to be filled by the lexical material. According to more recent theories, phrase structures are not generated by phrase structure rules, but are 'projected' from the lexical entries. The Projection Principle therefore obviates the need for phrase structure rules in the generative component.