Tree diagrams

The successive layers of constituents which make up a sentence can be shown most clearly on a tree diagram - so called because its branches resemble the branches of a tree. In a tree diagram, a basic sentence type at the top branches downwards in ever-increasing com­plexity (Figure 27).

The advantage of a tree diagram is that each join or node on the tree can be labelled, so that the whole construction becomes clearer (Figure 28).

S (sentence)

A family metaphor is used to refer to the relationships on a tree (Figure 29, page 65). A higher node is a mother, and the nodes on the branches immediately beneath her are her daughters. Daughters of the same mother are known as sisters. A mother is said to dominate the nodes beneath her. She immediately dominates her daughters,

but she also dominates her granddaughters, and great-granddaugh­ters, as it were

Rewrite rules

An alternative way of expressing the information found on a tree dia­gram is by means of rewrite rules. A rewrite rule is a replacement rule, in which the symbol to the left of an arrow is replaced by an expanded form written to the right of the arrow:

S –> NP VP means “Replace the symbol S by NP VP”.

VP –> V NP means “Replace the symbol VP by V NP”.

means 'Replace the symbol NP by D N .

NP->D N

The essential structure of The duck bit the burglar can therefore be summerised in just three rules:

S->NP VP

VP->V NP

NP->D N

On a three deagram, these three rules would appear as in Figure 30.

S

NP VP

V NP

D N Fig.30

These branching rules can then be supplemented by lexical substi­tution rules:

N -> duck, burglar V->bit D->the

The duck bit the burglar

The great advantage of rewrite rules is that they are perfectly explicit. They do not leave anything to the imagination. By following them, you could produce a perfect English sentence even if you did not know any English, since the rules are applied mechanically, step-by-step, one symbol at a time.

Note, however, that the above rewrite rules could also have resulted in the sentence:

The burglar bit the duck.

This does not matter, as the sequence is a perfectly good sentence of English (though admittedly a somewhat unlikely one). The rewrite rules are there to tell us what is a well-formed English sentence, not to give us information about the probable behaviour of burglars.

Identifying constituents

As we have seen, every sentence can be broken down into successive layers of constituents. However, not all sentences can be analysed with as little trouble as The duck bit the burglar. Consider the sentence: The mouse ran up the clock.

How should this be analysed? Should we bracket [ran up) together, on the assumption that these words could be replaced by a word such as climbed? Or should we bracket [up the clock] together, noting that the whole phrase could be replaced by a single word such as upwards? Problems of this type are solved by seeing whether the groups of words in question belong together as a constituent elsewhere, since words that are grouped together in one sentence are likely to recur as a single constituent in other sentences. One way of checking this is to construct sentences in which the original words occur in a different order: Up the clock ran the mouse. The mouse ran the clock up.

These sentences suggest that the words up the clock should be brack­eted together, since they can be moved as a chunk to the front of the sentence. We may therefore analyse the sentence as

[The mouse] [ran] [up the clock.) and draw the tree diagram as in Figure 32.

The sentence discussed above must be analysed differently from another, superficially similar sentence:

The mouse ate up the cheese. We can show the difference by switching the sentence around:* Up the cheese ate the mouse. (Compare: Dp the clock ran the mouse)

The mouse ate the cheese up. (Compare: *The mouse ran the clock up.)

We may therefore analyse the secord sentence as:

[The mouse] [ate up] [the cheese.]

and draw the tree diagram as in Figure 33, using the extra node-labels VB for 'phrasal verb' and PRT for 'particle':

Constituents behave in predictable ways, since languages ring the changes on a few recurring patterns. It is therefore possible to build up a store of specific 'tests' for the presence of a particular constituent in a given language. As up the clock suggests, one test for a PP (prepo­sition phrase = phrase containing a preposition) is that a preposition cannot immediately follow its NP. Just as you cannot say:

*The mouse ran the clock up,

so you cannot say: *Fenella went the woods into, * Doris swam the bridge under.

Let us now go on to consider this notion of 'tests' further, by consid­ering 'NP tests'.

NP tests

English NPs (noun phrases) recur in certain specifiable positions. Some of the main places in which they occur are:

1. At the beginning of a sentence before the verb: The cat ate the canary.

(b) At the end of a sentence after the verb: The canary feared the cat

3. After by in a passive sentence: The canary was eaten by the cat.

4. After an auxiliary verb in questions: Did the cat eat the canary?

Of course, other types of phrase can occur in some of these positions. But an NP such as the cat can occur in all of them. Consequently, if we find a phrase which we suspect might be an NP, we can apply these (and other) tests. For example, consider the sentences below: Uncle Harry kicked the cat. Suddenly Harry kicked the cat.

In order to find out whether the first two words in each sentence are an NP, we can apply the NP tests listed above:

(a) At the beginning of a sentence before a verb: Uncle Harry kicked the cat.

Suddenly Harry kicked the cat.

2. At the end of a sentence after a verb: The cat scratched Uncle Harry. *The cat scratched suddenly Harry.

3. After by in a passive sentence: The cat was kicked by Uncle Harry.

* The cat was kicked by suddenly Harry.

(d) After an auxiliary verb in questions: Did Uncle Harry kick the cat? *Did suddenly Harry kick the cat?

The failure of suddenly Harry to pass most of these NP tests shows that it cannot be an NP, whereas the success of Uncle Harry indicates that it probably is an NP.