The clause introduced by a relative pronoun is an adjective clause, since it serves as an adjective modifier of the antecedent. There are two different ways in which the antecedent may be thus modified.
Examples : A
1. The Italian, who wore a flower in his coat, smiled at me.
2. The Italian who wore a flower in his coat smiled at me.
In the first sentence, the italicized relative clause serves simply to describe the Italian, not to identify him. The flower is a mere detail of the picture.
In the second sentence, the relative clause serves not merely to describe the Italian, but also to distinguish him from all others. The flower is mentioned as a means of identification. The relative clause confines or restricts the meaning of the antecedent (Italian).
A relative pronoun that serves merely to introduce a descriptive fact is called a descriptive relative.
A relative pronoun that introduces a clause confining or limiting the application of the antecedent is called a restrictive relative.
Thus in Examples : A - 1, WHO is a descriptive relative.
In Examples : A - 2, WHO is a restrictive relative.
Before a descriptive relative we regularly make a pause in speaking, but never before a restrictive relative. Hence the rule….
A descriptive relative is preceded by a comma.
A restrictive relative is not preceded by a comma.
1. Three sailors, who were loitering on the pier, sprang to the rescue.
2. A clumsy weapon, which I took for a blunderbuss, hung over the fireplace.
3. I told the news to the first man that (or whom) I met.
4. The coins that (or which) you showed me are doubloons.
5. Nothing that I have ever read has moved me more profoundly than the third act of King Lear.
WHO, WHICH and THAT are all used as restrictive relatives.
But some writers prefer THAT to WHICH, especially in the nominative case.
Note : That is not now employed as a descriptive relative, though it was common in this use not very long ago. Thus in 1844 Disraeli wrote…“The deer, that abounded, lived here in a world as savage as themselves" (Coningsby, book iii, chapter 5).
The omission of the relative is possible only when the relative is restrictive.
1. The boy [whom] I saw at your house has left town. [Restrictive.]
2. Charles, whom I saw yesterday, had not heard the news. [Descriptive.]