Chomsky’s system of transformational grammar was substantially modified in 1965. Perhaps the most important modification was the incorporation, within the system, of a semantic component, in addition to the syntactic component and phonological component. (The phonological component may be thought of as replacing the morphophonemic component of Syntactic Structures.) The rules of the syntactic component generate the sentences of the language and assign to each not one but two structural analyses: a deep structure analysis as represented by the underlying phrase marker, and a surface structure analysis, as represented by the final derived phrase marker. The underlying phrase marker is assigned by rules of the base (roughly equivalent to the PS [Phrase-Structure] rules of the earlier system); the derived phrase marker is assigned by the transformational rules. The interrelationship of the four sets of rules is shown diagrammatically in Figure 7. The meaning of the sentence is derived (mainly, if not wholly) from the deep structure by means of the rules of semantic interpretation; the phonetic realization of the sentence is derived from its surface structure by means of the rules of the phonological component. The grammar (“grammar” is now to be understood as covering semantics and phonology, as well as syntax) is thus an integrated system of rules for relating the pronunciation of a sentence to its meaning. The syntax, and more particularly the base, is at the “heart” of the system, as it were: it is the base component (as the arrows in the diagram indicate) that generates the infinite class of structures underlying the well-formed sentences of a language. These structures are then given a semantic and phonetic “interpretation” by the other components.

The base consists of two parts: a set of categorial rules and a lexicon. Taken together, they fulfill a similar function to that fulfilled by the phrase-structure rules of the earlier system. But there are many differences of detail. Among the most important is that the lexicon (which may be thought of as a dictionary of the language cast in a particular form) lists, in principle, all the vocabulary words in the language and associates with each all the syntactic, semantic, and phonological information required for the correct operation of the rules. This information is represented in terms of what are called features. For example, the entry for “boy” might say that it has the syntactic features: [+ Noun], [+ Count], [+ Common], [+ Animate], and [+ Human]. The categorial rules generate a set of phrase markers that have in them, as it were, a number of “slots” to be filled with items from the lexicon. With each such “slot” there is associated a set of features that define the kind of item that can fill the “slot.” If a phrase marker is generated with a “slot” for the head of a noun phrase specified as requiring an animate noun (i.e., a noun having the feature [+ Animate]), the item “boy” would be recognized as being compatible with this specification and could be inserted in the “slot” by the rule of lexical substitution. Similarly, it could be inserted in “slots” specified as requiring a common noun, a human noun, or a countable noun, but it would be excluded from positions that require an abstract noun (e.g., “sincerity”) or an uncountable noun (e.g., “water”). By drawing upon the syntactic information coded in feature notation in the lexicon, the categorial rules might permit such sentences as “The boy died,” while excluding (and thereby defining as ungrammatical) such nonsentences as “The boy elapsed.”

One of the most controversial topics in the development of transformational grammar was the relationship between syntax and semantics. Scholars working in the field agreed that there is a considerable degree of interdependence between the two, and the problem was how to formalize this interdependence. One school of linguists, called generative semanticists, accepted the general principles of transformational grammar but challenged Chomsky’s conception of deep structure as a separate and identifiable level of syntactic representation. In their opinion, the basic component of the grammar should consist of a set of rules for the generation of well-formed semantic representations. These would then be converted by a succession of transformational rules into strings of words with an assigned surface-structure syntactic analysis, there being no place in the passage from semantic representation to surface structure identifiable as Chomsky’s deep structure. Chomsky himself denied that there is any real difference between the two points of view and has maintained that the issue is purely one of notation. That this argument could be put forward by one party to the controversy and rejected by the other is perhaps a sufficient indication of the uncertainty of the evidence. Of greater importance than the overt issues, in so far as they are clear, was the fact that linguists were now studying much more intensively than they had in the past the complexities of the interdependence of syntax, on the one hand, and semantics and logic, on the other.

The role of the phonological component of a generative grammar of the type outlined by Chomsky is to assign a phonetic “interpretation” to the strings of words generated by the syntactic component. These strings of words are represented in a phonological notation (taken from the lexicon) and have been provided with a surface-structure analysis by the transformational rules (see Figure 7). The phonological elements out of which the word forms are composed are segments consisting of what are referred to technically as distinctive features (following the usage of the Prague school, see below The Prague school). For example, the word form “man,” represented phonologically, is composed of three segments: the first consists of the features [+ consonantal], [+ bilabial], [+ nasal], etc.; the second of the features [+ vocalic], [+ front], [+ open], etc.; and the third of the features [+ consonantal], [+ alveolar], [+ nasal], etc. (These features should be taken as purely illustrative; there is some doubt about the definitive list of distinctive features.) Although these segments may be referred to as the “phonemes” /m/, /a/, and /n/, they should not be identified theoretically with units of the kind discussed in the section on Phonology under Structural linguistics. They are closer to what many American structural linguists called “morphophonemes” or the Prague school linguists labelled “archiphonemes,” being unspecified for any feature that is contextually redundant or predictable. For instance, the first segment of the phonological representation of “man” will not include the feature [+ voice]; because nasal consonants are always phonetically voiced in this position in English, the feature [+ voice] can be added to the phonetic specification by a rule of the phonological component.

One further important aspect of generative phonology (i.e., phonology carried out within the framework of an integrated generative grammar) should be mentioned: its dependence upon syntax. Most American structural phonologists made it a point of principle that the phonemic analysis of an utterance should be carried out without regard to its grammatical structure. This principle was controversial among American linguists and was not generally accepted outside America. Not only was the principle rejected by the generative grammarians, but they made the phonological description of a language much more dependent upon its syntactic analysis than has any other school of linguists. They claimed, for example, that the phonological rules that assign different degrees of stress to the vowels in English words and phrases and alter the quality of the relatively unstressed vowel concomitantly must make reference to the derived constituent structure of sentences and not merely to the form class of the individual words or the places in which the word boundaries occur.