T I WANT is that red-backed history_,
the restrictive relative clause is, _that I want_, and limits the
application of _book_.

26. _Who_ and _which_ are known as the EXPLANATORY or NON-RESTRICTIVE
RELATIVES, and should be used ordinarily only to introduce relative
clauses which add some new thought to the author's principal thought.
_Spanish, WHICH IS THE LEAST COMPLEX LANGUAGE, is the easiest to
learn_. In this sentence the principal thought is, _Spanish is
the easiest language to learn_. The relative clause, _which is
the least complex language_, is a thought, which, though not fully
so important as the principal thought, is more nearly cooerdinate
than subordinate in its value. It adds an additional thought of
the speaker explaining the character of the Spanish language. When
_who_ and _which_ are thus used as explanatory relatives, we see
that the relative clause may be omitted without making the sentence
incomplete.

Compare the following sentences:

Explanatory relative clause: That book, _which is about history_,
has a red cover.

Restrictive relative clause: The book _that is about history_ has
a red cover.

Explanatory relative clause: Lincoln, _who was one of the world's
greatest men_, was killed by Booth.

Restrictive relative clause: The Lincoln _that was killed by Booth_
was one of the world's greatest men.

NOTE.--See Sec.111, for rule as to the punctuation of relative clauses.

27. INTERROGATIVE PRONOUNS. An Interrogative Pronoun is a pronoun
used to ask a question. The interrogative pronouns are, _who_ (_whose,
whom_), _which_, and _what_. In respect to antecedents, _who_ should
be used only in reference to persons; _which_ and _what_ may be
used with any antecedent, persons, animals, or things.

EXERCISE 10

_Choose the proper relative or interrogative pronoun to be inserted
in each of the following sentences. Insert commas where they are
needed._ (_See_ Sec.111):

1. The kindly physician ---- was so greatly loved is dead.
2. This is the man ---- all are praising.
3. John ---- is my coachman is sick.
4. The intelligence ---- he displayed was remarkable.
5. Intelligence ---- he had hitherto not manifested now showed its
presence.
6. He maintains that the book ---- you used is now ruined.
(Does _which_ or _that_ have the more pleasing sound here?)
7. The pleasure ---- education gives the man ---- has it is a
sufficient reward for the trouble ---- it has cost