In Notes 1 and Assignment 1, we saw that the interpretation of three types of noun phrases (reflexive pronouns, personal pronouns, and full noun phrases) is constrained by their structural relations with other noun phrases. The question that we will address over the next few weeks is: What are the origins of these structural relations, and, for that matter, of syntactic structure itself? The present notes discuss two approaches to this question that have been pursued in the history of generative grammar. The first, based on so-called phrase structure rules, characterized the field from its beginnings until roughly the early 1980s. It had a number of conceptual shortcomings, however, which over time led to its rejection and to the adoption of the second approach, according to which syntactic structure is projected from the lexicon.
| (1) | a. | VP ---> V NP NP | |
| b. | VP ---> V NP PP | ||
| c. | VP ---> V NP | ||
| d. | VP ---> V |
From a mathematical point of view, such rules are part of a so-called context-free grammar, and they therefore have–by definition–certain formal properties. Specifically, the lefthand side of a phrase structure rule must consist of a single symbol, whereas the righthand side of a phrase structure rule may consist of one or more symbols.
As they stand, phrase structure rules are devices that manipulate strings without regard for structure. For instance, given the two rules in (2), the string in (3a) can be successively rewritten as (3b,c).
| (2) | a. | VP ---> V NP | |
| b. | NP ---> Article N |
| (3) | a. | VP | ||
| b. | V NP | by (2a) | ||
| c. | V Article N | by (2b) |
Although the phrase structure rule for NP in (2b) implies that the article and the noun form a structural unit, nothing in the result of the rewriting (= (3c)) reflects that fact. That is, (3c) contains no indication that 'Article' and 'N' belong together more closely than do 'V' and 'Article'. In order to retain the structural information inherent in the phrase structure rules, it is therefore necessary to keep track of how each symbol is rewritten. This can be done by following the algorithm in (4).
| (4) | a. | The symbol on the lefthand side of a rule corresponds to a node. | |
| b. | The symbol(s) on the righthand side of the rule correspond to a set of nodes in the same linear order as that of the symbols in the rule. | ||
| c. | The rewrite arrow corresponds to a mother-daughter relation between the symbols from the lefthand and righthand sides of the rule. The mother-daughter relationship is indicated by downward-pointing branches between the mother and its daughters. |
Using (4), the rules in (1) and (2) can be translated into the trees in (5).
| (5) | a. |
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In the mathematical literature, the syntactic trees that result from applying (4) are known as derivation trees because they represent the way that a particular sequence of symbols (say, 'V Article N') is derived from an original symbol (say, 'VP').
The phrase structure rules in (1) and (2) contain only nonterminal symbols (= syntactic categories; roughly speaking, parts of speech). In order to generate phrases and sentences consisting of actual words, there must also be rules available whose righthand side contains terminal symbols, like those in (6). In contrast to nonterminals, terminal symbols cannot appear on the lefthand side of a phrase structure rule. This is what gives them their name; since they cannot be rewritten, they terminate the particular bit of the derivation involving them.
| (6) | a. | V ---> tell | |
| b. | V ---> put | ||
| c. | V ---> devour | ||
| d. | V ---> waited |
Because we can think of the terminal symbols–the lexical items–as being inserted into structures like those in (5), rules like (6) are known as lexical insertion rules. Lexical insertion rules are like ordinary phrase structure rules in that the lefthand side in both must be a single nonterminal symbol. However, the righthand side of a lexical insertion rule is constrained to be a single terminal symbol.
| (7) | a. | 
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In conjunction with other phrase structure rules, it then becomes possible to build structures for sentences like those in (8).
| (8) | a. | I will tell you the answer. | |
| b. | They put the book on the shelf. | ||
| c. | The lion will devour the wildebeest. | ||
| d. | They waited. |
Although combining phrase structure and lexical insertion rules correctly allows us to build structures for the sentences in (8), a serious problem with it was noted early on: it also incorrectly gives structures for the sentences in (9).
| (9) | a. | * | I will put you the answer. |
| b. | * | They waited the book on the shelf. | |
| c. | * | They devoured. |
In traditional terms, the difficulty is that the system of phrase structure rules and lexical insertion rules fails to distinguish among various subcategories of verbs, such as intransitive, transitive and ditransitive.
We will define intransitive, transitive, and ditransitive verbs as taking zero, one, and two complements, respectively. The term 'complement' is defined in the section Complements versus adjuncts.Transformational grammarians therefore proposed to incorporate this sort of information into each verb's lexical entry. The idea is that each lexical item has an entry in our mental grammar, comparable to a conventional dictionary entry (though much more detailed). Included in the lexical entry is the lexical item's pronunciation, its meaning, its syntactic category, and, crucially for present purposes, the syntactic environments in which it can occur. When represented as in (10), these environments are known as subcategorization frames. The blank line represents the position of the verb, and the remaining syntactic categories represent the verb's environment. As (10a,e) show, verbs may be associated with more than one subcategorization frame.
| (10) | a. | tell | ___, ___ NP NP, ___ NP PP | |
| b. | put | ___ NP PP | ||
| c. | devour | ___ NP | ||
| d. | waited | ___ | ||
| e. | eat | ___, ___ NP |
Subcategorization frames are used as follows. At the point of lexical insertion, a verb's subcategorization frame is checked against the syntactic environment that the verb is being inserted into. If the environment matches the frame, lexical insertion goes forward, but if not, lexical insertion fails. Thus, the sentences in (8) are generated, but the ones in (9) are not.
First, the architecture of a grammar that is based on phrase structure rules and subcategorization frames requires the information in each subcategorization frame to duplicate information in some phrase structure rule. The history of generative grammar has been driven by the assumption that such redundancy indicates a failure of insight, and that more insight will be achieved by finding a way to eliminate the redundancy.
Second, nothing in the mathematical constraints on phrase structure rules prohibits 'crazy' rules like those in (11).
| (11) | a. | NP ---> V Adj | |
| b. | VP ---> Adj |
But structures corresponding to such rules are simply not found in the world's languages. Rather, a phrase of a particular type, say a verb phrase, contains a lexical item of that same type, in this case, a verb. That is, phrases have a core, which in traditional grammar is called the head–a term which has been adopted by generative grammarians.
A first important piece of information that needs to be represented in a treelet is a lexical item's syntactic category. For the moment, we will focus on the four so-called lexical categories: N(oun), V(erb), A(djective) and P(reposition). We can easily represent a lexical item's syntactic category by representing it as the lexical item's mother, as shown in (12).
| (12) | a. | 
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A second piece of information that needs to be represented is the fact that lexical items serve as heads of phrases. Again, we can represent this by having an appropriate type of phrase dominate the lexical item. We can actually think of the representation as being generated in two steps: first, a phrasal node of as yet indeterminate type is added to the treelets in (12), and then the lower syntactic category's type percolates (to use a widespread metaphor) to determine the type of the entire phrase.
| (13) | a. | 
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| (14) | a. | 
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We may think of the simple structures in (14), consisting of a lexical item, its syntactic category and a phrase corresponding to the category, as the irreducible core of a treelet, its spine. In many cases, a treelet also contains further slots for various other syntactic dependents. This permits the straightforward representation of the various subcategories of verbs, the fact that prepositions take objects, and so on. Some examples of treelets for prepositions and ditransitive, transitive and intransitive verbs are shown in (15).
| (15) | a. | 
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Notice that the slots in the treelets other than the lexical item anchoring it are empty. The reason is that each treelet is intended to provide exactly the information that is characteristic of the lexical item anchoring it–no less, but also no more. For instance, a verb like devour requires an object, but not a particular one; many different phrases will fill the bill, just as long as they are noun phrases.
A final issue concerns verbs such as eat, which can be used either with or without a following object. In the phrase structure approach to generating syntactic structure, this fact is represented by associating multiple subcategorization frames with a single lexical item; see (10e). The notational counterpart in the current projection approach is to associate a single lexical item with more than one treelet, as in (16).
| (16) | a. | 
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As we will see in a moment, the structures in (14)-(16) are oversimplified, but for now, they illustrate how it is possible to reconstruct trees like those in (7) without incurring the redundancy inherent in a system of phrase structure rules, lexical insertion rules and subcategorization frames.
Note: For expository convenience, we focus in what follows on heads that are verbs. The discussion is extended to other types of heads in Notes 3 and 4.
There are a number of conceivable ways that one could go about adding such a slot. The simplest idea seems to be to add a leftmost daughter to the VP nodes in (15b-d), yielding structures as in (17).
| (17) | a. | 
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Notice that in such 'flat' structures, the subject and any objects of the verb c-command each other. But we know from the distribution of reflexive pronouns that subjects asymmetrically c-command objects (recall Assignment 1, Exercise 2). That is, subjects c-command objects, but objects don't c-command subjects. Since the representations in (17) fail to represent this fact, they must be rejected.
What is necessary is a node that groups the verb together with any objects, but excludes the subject, as in (18). The required additional node is standardly called V' (read as V-bar).
| (18) | a. | 
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Notice that we have no direct evidence yet for the existence of a V' in the treelets for intransitive verbs such as waited. For the moment, we assume its existence on conceptual grounds: including it makes the treelet in (18c) analogous to those in (18a,b).
Note: An empirical argument for including V' in the treelets of intransitive verbs is explored in Assignment 2, Exercise 1.
Notice further that it is V' and not the node labelled VP that corresponds to the 'verb phrase' of traditional grammar. Although the terminology of traditional grammar and generative grammar often coincides, this is not always the case. Such a divergence of technical from more colloquial usage is nothing out of the ordinary; it occurs in the development of any science or technical discipline.
We now introduce some useful terminology to discuss hierarchically structured treelets as in (18). We say that the lexical item projects the syntactic structure in the treelet. The lexical item's syntactic category, the head of the projected structure–here, V–is also referred to as the lexical projection. V' is the intermediate projection, and VP is the phrasal projection or maximal projection. We extend the notion of spine introduced earlier to include all three of these projections. The three projections are said to have distinct bar levels, as summarized in (19).
| (19) | Projection | Bar level | Example | ||
| Lexical | 0 | V | |||
| Intermediate | 1 | V' | |||
| Maximal, phrasal | 2 | VP | |||
The sister of the intermediate projection is called the specifier; it is the slot for subjects. In English, the specifier of VP is the intermediate projection's left sister, but in a VOS language like Malagasy, it is the right sister. Following traditional terminology, a head's sister is called a complement. There is only one specifier, but there may be more than one complement. Taken together, the subject and any complements are often referred to as a head's arguments.
| (20) | They eat [NP five apples] [NP every day] . |
As mentioned earlier, certain verbs–among them, eat–are associated with more than one treelet. The grammaticality of (20) might therefore be taken to indicate that eat is associated with the ditransitive treelet in (21) (in addition to the intransitive and transitive treelets in (16)).
| (21) | 
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But however plausible this suggestion seems at first glance, there turns out to be compelling evidence against it. This evidence comes from a syntactic phenomenon by the name of do so substitution, which is illustrated in (22) and (23). As is evident, do so can substitute either for a complete V', as in (22b), or for part of one, as in (23b).
| (22) | a. | They eat five apples every day, ... | |
| b. | and we do so, too. | ||
| (23) | a. | They eat five apples every day, ... | |
| b. | and we do so every week. |
Let us now make an assumption that has been standard in syntactic theory from before the times of generative grammar–namely, that substitution is possible iff the sequence of words being replaced is a syntactic constituent (= unit of syntactic structure). In trees, constituents are represented as nodes that exhaustively dominate the sequence in question.
| (24) | exhaustively dominate: A node exhaustively dominates a sequence of symbols iff it dominates all and only the symbols in question (neither more nor less). |
For instance, A dominates the sequence B C in (25a-c), but exhaustively dominates it only in (25a). A doesn't exhaustively dominate B C in (25b,c) because it dominates too much material. A also fails to exhaustively dominate B C in (25d) because it dominates too little material. Domination is a necessary condition for exhaustive domination, but not a sufficient condition, as (25b,c) shows.
| (25) | a. | 
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The question arises whether the representation in (21) is consistent with the do so substitution facts in (22) and (23). The answer is that (21) is consistent with do so substitution in (22b), since the sequence being replaced is exhaustively dominated by V'. But (21) is not consistent with the grammaticality of do so substitution in (23b), because it contains no node that exhaustively dominates the sequence eat five apples. Since there is no reason to assume that the verb phrases in (22) and (23) have distinct structures, the treelet in (21) must be rejected.
But what then is the correct syntactic structure for the verb phrase eat five apples every day? It must be the case that both the entire V' is a constituent (= exhaustively dominated by a single node), as in (21), but also that the subsequence eat five apples is a constituent. The tree in (26) has the proper shape.
| (26) | 
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Notice that in (26), the NP five apples is a sister of the head V, whereas the NP every day and the head V are more distantly related. Given our definition earlier of complements as sisters of heads, only the direct object is a complement. But what relation does every day bear to the head V then, being neither a complement nor a specifier? Again adopting a term from traditional grammar, we refer to constituents related to a head as the NP every day is to V in (26) as modifiers or adjuncts.
It is worth noting that given the phrase structure representations developed so far, the dependent elements we have been discussing–complements, adjuncts and specifiers–all stand in distinct structural relations to the head (and to the spine more generally). Both complements and adjuncts are daughters of intermediate projections, but they differ crucially in that complements are sisters of heads, whereas adjuncts are sisters of the next higher projection level, intermediate projections. As sisters of intermediate projections, adjuncts resemble specifiers. But once again, the two relations are distinct because adjuncts are daughters of intermediate projections, whereas specifiers are daughters of maximal projections. These structural distinctions are summarized in (27).
| (27) | Linguistic relation to head | Sister of ... | Daughter of ... | ||
| Complement | Head | Intermediate projection | |||
| Adjunct | Intermediate projection | Intermediate projection | |||
| Specifier | Intermediate projection | Maximal projection | |||
Both traditional and generative grammar maintain that the relation between heads and their arguments is closer than that between heads and adjuncts. The idea is that adjuncts are not required by a particular lexical item; rather, they are optional specifications. Now we have stated repeatedly that treelets are intended to provide exactly the information that is characteristic of the lexical item anchoring it–no less, but also no more. In the approach to phrase structure that we adopt here, therefore, treelets include slots for arguments (= subjects and any complements), but not for adjuncts. This means that the treelet for eat that is used to generate the syntactic structure for a sentence like (20) has only a single V', as in (28)–not two, as in (26).
| (28) | 
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It is clear that the syntactic structure of a sentence like They eat five apples can be derived by building subtrees for they and five apples and then substituting them in the appropriate slots. This process of substitution is illustrated for the complement node in (29). The empty complement node in the containing syntactic structure in (29a) is the target of substitution and is replaced with the tree for the complement in (29b). The result of the substitution is shown in (29c). The analogous process of substituting they for the empty specifier slot in (29a) results in the tree for the entire sentence.
| (29) | a. |
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But given the absence of adjunct slots, how can adjuncts enter into syntactic structure? The answer is that adjuncts are integrated into treelets via a separate two-step process called adjunction. We illustrate the adjunction process with the treelet for eat from (28), repeated as (30a), and the tree for the adjunct phrase every day, schematically given in (30b). The target of adjunction is the intermediate projection in the treelet for eat.
| (30) | a. |
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The first step in adjunction is to make a copy of the target of adjunction right above the original node, as in (31a). The second step is to attach the tree for the adjunct phrase as a daughter of the newly created node, as in (31b).
Note: It is important to attach the modifier as a daughter of the higher copy of the intermediate projection. Attaching the modifier as a daughter of the lower copy would result in a tree that gives the erroneous impression that the attached constituent is a complement.
| (31) | a. | 
| b. | 
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The final result of adjunction in (31b) is identical to the tree in (26) precisely the desired result.