The groups of words we have been considering in Sentences 11 to 13-3 are either restrictive or explanatory groups, with the functions of nouns, adjectives, or adverbs, and with the relations that these parts of speech take in the construction of language.
The meanings of the terms “restrictive” and “explanatory” will appear as we consider other sentences; and the differentiation in the punctuation of restrictive and explanatory groups will be plain.
Our next sentence will serve a twofold purpose: first, to show how difficult it is to punctuate a sentence out of its context; and, secondly, to show that a sentence may be given two meanings by punctuation:
14. The boy who is at home is my best pupil.
14-1. The boy, who is at home, is my best pupil.
The person who is thoroughly familiar with the reasons for the use of marks can interpret these sentences; and he can also construct a context requiring the commas or their omission. On the other hand, one not familiar with such reasons could probably do neither.
The value of this knowledge is quite inestimable. Because of ignorance of it on the part of legislators, our courts have been required to determine the meanings of municipal, state, and national laws involving vital social relations and vast financial interests.
Applying the general principles already discussed, we say that the first comma in No. 14-1 is to show that the relation between “boy” and “who is my best pupil” is not the same relation that exists between the same groups in No. 14. But why is this? Let us construct contexts for the sentences, and then study them in the light of the information thus obtained.
Suppose a visitor to a school asks the teacher about a certain class, and the teacher replies as follows:
14-2. The class is composed of six boys. The boy who is at home is my best pupil.
The group of words “who is at home” is an adjective; and the meaning of the noun with the qualification made by the adjective may be thus expressed: the at-home boy. In this form the group specifies what boy, and so restricts the boy named as to mean a certain, definite boy.