When two or more words are combined to form a new concept with a new meaning, we talk about a compound.

A compound can be a combination of a noun and a noun (school nurse), an adjective and a noun (full noon), an adverb and a verb (far-reaching), a verb and an adverb (check-up), a preposition and a noun (underworld), a preposition and a verb (overestimate), an adjective and another adjective (blue-green) – and a few more combinations.

In English there are three ways to write compound words: as separate words (open compounds), as one word (closed compounds) or as words combined with a hyphen (hyphenated compounds). This can sometimes be rather confusing. We write head office as separate words, we write head-teacher with a hyphen and we write headmaster as one word. Likewise we have table knife but tablespoon.

This may seem frustrating, and if you are in doubt, you had better check a dictionary or a style sheet. (Actually, you can also write stylesheet. Some compounds can take any of the three forms. You can write life style, life-style or lifestyle.)

OPEN COMPOUNDS

Unlike some other languages – German, Swedish or Finnish, for example – English often does not combine the separate words into one word. A breakfast table is in German Frühstückstisch; a hotel room is in Swedish hotellrum; a taxi driver is in Finnish taksinkuljettaja.

Here are some examples of open compounds in English:

apple pie	half sister
coffee mug	information technology
computer network	light year
couch potato	living room
database design	master bedroom
decision maker	orange juice
dinner table	post office
English teacher	swimming pool
evening dress	truck driver
football stadium	video game
full moon	washing machine

CLOSED COMPOUNDS

The following are examples of compounds written in one word:

afternoon	makeup
airport	newspaper
blackboard	notebook
bodyguard	online
bookstore	paycheck
cupcake	policewoman
cowboy	skateboard
doorbell	substandard
downtown	takeaway
football	textbook
grandmother	underworld
handout	wallpaper
headache	watermelon
input	worksheet

Compounds with words from Latin or Greek are written as one word:

photography

agriculture

HYPHENATED COMPOUNDS

Many compounds – especially those formed by two nouns – used to be hyphenated, but now most of them are written either as one word or two separate words. Here are some compounds that are still hyphenated (and you will notice that they are generally not of the noun+noun type):

broad-minded	runner-up
check-in	secretary-general
dry-cleaning	self-esteem
far-reaching	six-pack
go-between	well-being
passer-by	X-ray

Compounds with three or more words are usually written with hyphens:

around-the-clock	merry-go-round
do-it-yourself	mother-of-pearl
editor-in-chief	right-of-way
father-in-law	jack-of-all-trades
happy-go-lucky	state-of-the-art

Compound numbers from twenty-one to ninety-nine are hyphenated:
My dad is forty-two.

Fractions also take a hyphen:
We had already driven two-thirds of the way.
Less than one-fifth of the operators are women.

But with a instead of one there is no hyphen: Less than a fifth of the operators are women.

Some compounds have changed from being hyphenated to a single word. We used to write on-line and world-wide, but nowadays online and worldwide are more common. This also applies to words such as cooperate and proactive.

To avoid confusion, a hyphen is used when the prefix ends and the base word begins with the same vowel:

anti-intellectual

COMPUNDS AS MODIFIERS

When compounds are used as modifiers, they are written with a hyphen. A modifier works as an adjective or adverb to add information about the word directly following it.

A six-year-old boy.
But: The boy was six years old.

We rely on just-in-time delivery.
But: I arrived just in time. The clock struck three when I opened the door.

You can read about compounds as modifiers here.