What Bloomfield had in mind as the criterion for form class membership (and therefore of syntactic equivalence) may best be expressed in terms of substitutability. Form classes are sets of forms (whether simple or complex, free or bound), any one of which may be substituted for any other in a given construction or set of constructions throughout the sentences of the language.
The smaller forms into which a larger form may be analyzed are its constituents, and the larger form is a construction. For example, the phrase "poor John" is a construction analyzable into, or composed of, the constituents "poor" and "John." Because there is no intermediate unit of which "poor" and "John" are constituents that is itself a constituent of the construction "poor John," the forms "poor" and "John" may be described not only as constituents but also as immediate constituents of "poor John." Similarly, the phrase "lost his watch" is composed of three word forms--"lost," "his," and "watch"--all of which may be described as constituents of the construction. Not all of them, however, are its immediate constituents. The forms "his" and "watch" combine to make the intermediate construction "his watch"; it is this intermediate unit that combines with "lost" to form the larger phrase "lost his watch." The immediate constituents of "lost his watch" are "lost" and "his watch"; the immediate constituents of "his watch" are the forms "his" and "watch." By the constituent structure of a phrase or sentence is meant the hierarchical organization of the smallest forms of which it is composed (its ultimate constituents) into layers of successively more inclusive units. Viewed in this way, the sentence "Poor John lost his watch" is more than simply a sequence of five word forms associated with a particular intonation pattern. It is analyzable into the immediate constituents "poor John" and "lost his watch," and each of these phrases is analyzable into its own immediate constituents and so on, until, at the last stage of the analysis, the ultimate constituents of the sentence are reached. The constituent structure of the whole sentence is represented by means of a tree diagram in Figure 1.
Each form, whether it is simple or composite, belongs to a certain form class. Using arbitrarily selected letters to denote the form classes of English, "poor" may be a member of the form class A, "John" of the class B, "lost" of the class C, "his" of the class D, and "watch" of the class E. Because "poor John" is syntactically equivalent to (i.e., substitutable for) "John," it is to be classified as a member of B. So too, it can be assumed, is "his watch." In the case of "lost his watch" there is a problem. There are very many forms--including "lost," "ate," and "stole"--that can occur, as here, in constructions with a member of B and can also occur alone; for example, "lost" is substitutable for "stole the money," as "stole" is substitutable for either or for "lost his watch." This being so, one might decide to classify constructions like "lost his watch" as members of C. On the other hand, there are forms that--though they are substitutable for "lost," "ate," "stole," and so on when these forms occur alone--cannot be used in combination with a following member of B (cf. "died," "existed"); and there are forms that, though they may be used in combination with a following member of B, cannot occur alone (cf. "enjoyed"). The question is whether one respects the traditional distinction between transitive and intransitive verb forms. It may be decided, then, that "lost," "stole," "ate" and so forth belong to one class, C (the class to which "enjoyed" belongs), when they occur "transitively" (i.e., with a following member of B as their object) but to a different class, F (the class to which "died" belongs), when they occur "intransitively." Finally, it can be said that the whole sentence "Poor John lost his watch" is a member of the form class G. Thus the constituent structure not only of "Poor John lost his watch" but of a whole set of English sentences can be represented by means of the tree diagram given in Figure 2. New sentences of the same type can be constructed by substituting actual forms for the class labels.
Any construction that belongs to the same form class as at least one of its immediate constituents is described as endocentric; the only endocentric construction in the model sentence above is "poor John." All the other constructions, according to the analysis, are exocentric. This is clear from the fact that in Figure 2 the letters at the nodes above every phrase other than the phrase A + B (i.e., "poor John," "old Harry," and so on) are different from any of the letters at the ends of the lower branches connected directly to these nodes. For example, the phrase D + E (i.e., "his watch," "the money," and so forth) has immediately above it a node labelled B, rather than either D or E. Endocentric constructions fall into two types: subordinating and coordinating. If attention is confined, for simplicity, to constructions composed of no more than two immediate constituents, it can be said that subordinating constructions are those in which only one immediate constituent is of the same form class as the whole construction, whereas coordinating constructions are those in which both constituents are of the same form class as the whole construction. In a subordinating construction (e.g., "poor John"), the constituent that is syntactically equivalent to the whole construction is described as the head, and its partner is described as the modifier: thus, in "poor John," the form "John" is the head, and "poor" is its modifier. An example of a coordinating construction is "men and women," in which, it may be assumed, the immediate constituents are the word "men" and the word "women," each of which is syntactically equivalent to "men and women." (It is here implied that the conjunction "and" is not a constituent, properly so called, but an element that, like the relative order of the constituents, indicates the nature of the construction involved. Not all linguists have held this view.)
One reason for giving theoretical recognition to the notion of constituent is that it helps to account for the ambiguity of certain constructions. A classic example is the phrase "old men and women," which may be interpreted in two different ways according to whether one associates "old" with "men and women" or just with "men." Under the first of the two interpretations, the immediate constituents are "old" and "men and women"; under the second, they are "old men" and "women." The difference in meaning cannot be attributed to any one of the ultimate constituents but results from a difference in the way in which they are associated with one another. Ambiguity of this kind is referred to as syntactic ambiguity. Not all syntactic ambiguity is satisfactorily accounted for in terms of constituent structure.
