Words indicating number are called numerals. They are adjectives, nouns or adverbs.

1. There are seven days in the week. [Adjective]
2. Twelve make a dozen. [Noun]
3. I have called twice. [Adverb]

The chief classes of numerals are cardinals and ordinals.

Cardinal numeral adjectives (one, two, three, four, etc.) are used in counting and answer the question “How many?” Here are few examples.

1. I had to pay three dollars.

2. There were forty-two vessels in the fleet.

Note : In such expressions as THE BOY WAS SIXTEEN, the numeral is a predicate adjective limiting boy. We need not expand sixteen to SIXTEEN YEARS OLD.

Ordinal numeral adjectives (first, second, third, etc.) denote the position or order of a person or thing in a series. Here are few examples.

1. Carl plays the second violin.

2. Your friend is sitting in the fifth row.

All the cardinal and ordinal numerals may become nouns and may take a plural ending in some of their senses. Here are few examples.<

1. One is enough.
2. Four are missing.
3. The nine played an excellent game.
4. Three twos are six.
5. The men formed by fours.
6. Thousands perished by the way.
7. Eight is two thirds of twelve. [So regularly in fractional parts.]

Note : Hundred, thousand, million were originally nouns, but are now equally common as adjectives. Other numeral nouns are twain, couple, pair, brace, trio, quartette, quintette, foursome, dozen, score, century.

Certain numeral adjectives (single, double, triple, etc.) indicate how many times a thing is taken or of how many like parts it consists. Here are few examples.<

1. A double row of policemen stood on guard.
2. A fourfold layer of chilled steel forms the door.

Some of these words may be used as adverbs. Here are few examples.<

1. The cabman charged double.
2. His fear increased tenfold.

Certain numeral adverbs and adverbial phrases indicate how many times an action takes place. Here are few examples.

1. Once my assailant slipped.

2. I rang the bell twice.

3. The river hath thrice flow’d, no ebb between. - Shakespeare

The only adverbs of this kind in ordinary use are once and twice. For larger numbers an adverbial phrase (three times, four times, etc.) is employed. Thrice, however, is still common in poetry and the solemn style.

Grammar Index