Compound Nouns

Compound nouns are nouns that are formed out of one or more common nouns or other parts of speech and make up a relatively frequent type of noun in the English language. Two examples include the following:

seafood

thunderstorm

In many cases, they are formed with little or no apparent relation to the root words that constitute them, which can sometimes be noted in their pronunciation.

Compound nouns are often confused with noun phrases. The two can be distinguished in that compound nouns form entirely new objects or concepts where the root words are inseparable, whereas noun phrases are formed by a noun with one or more modifiers (usually adjectives) which are separable. For example, the following word is a compound noun:

air conditioner

But the following is a noun phrase:

old man

There are three forms of compound nouns, namely: (1) closed, (2) hyphenated, and (3) open form compound nouns. You will find an explanation of each below:

Closed Form

Closed form compound nouns are normally the easiest compound nouns to distinguish. They are formed when two or more common nouns are joined together to form a single new word without being punctuated by a space.

Typically, closed form compound nouns contain some relation to the original root words in terms of their meaning. However, it also possible for them to have no apparent relation to the root words.

Below are several examples of closed form compound nouns:

earthquake	earth + quake
teapot	tea + pot
breakfast	break + fast
software	soft + ware
butterfly	butter + fly
tablecloth	table + cloth

Hyphenated Form

Hyphenated compound nouns are comprised of two or more separate words that are written together using a hyphen. Of the three forms of compound nouns, the root words making up hyphenated compound nouns are the most likely to have an apparent relation to the compound word with respect to meaning.

Here are several examples of hyphenated compound nouns:

light-year	light + year
mother-in-law	mother + in + law
sing-along	sing + along
self-esteem	self + esteem
dry-clean	dry + clean

Open Form

As we noted earlier, open form compound nouns are typically the most difficult form of compound nouns to distinguish as they can be easily be confused with noun phrases.

Normally, open form compound nouns use two or more words to describe a single object which contains the essence of the constituent root words. Open form compound nouns are often new words that are used to describe inventions, ideas, or concepts.

Here are several examples of open form compound nouns:

ice cream	ice + cream
full moon	full + moon
wine glass	wing + glass
coffee mug	coffee + mug
high school	high + school
jet plane	jet + plane

Evolution of Compound Noun Forms

Whether a compound noun form is considered closed, open, or hyphenated depends on the prevailing writing and spelling conventions, which can change over time.

As the English language is constantly evolving, a typical evolution in compound noun forms is that they tend to begin in the open form, then over time become hyphenated, before eventually becoming accepted in closed form.

During transitions, it is common for some compound nouns to be accepted in more than one form, e.g. the word light-year is acceptable in either hyphenated form or in open form (i.e. light year).