Compounds or compound words are words that are derived from two separate words joined together. Most compound words are nouns, composed of an adjective and a noun or two nouns. There are three different types of compound words in the English language: closed compound words, hyphenated compound words and open compound words.

An open compound word is a noun that is composed of two words that are often used together, yet still maintain a space between the two words. This type of compound is also referred to as a spaced compound word. It might be difficult to identify such a term as a compound word. To qualify as an open compound word, the term must have a different meaning from the definitions of each of the original words. For instance, the open compound word swimming pool is composed of the word swimming, which means moving one’s body through water, and pool, which means a body of still water. However, when used together in the word swimming pool, the definition is a manmade body of water, filtered and cleaned, that a person may play, float or swim in. Open compound words are often the most recently coined compound words.
A hyphenated compound word is a compound that is composed of two or more words linked by hyphens. Hyphenated compound words are the most likely type of compound word to be composed of two adjectives or two verbs. Hyphenated compound words are often coined by writers, as J.K. Rowling did when she created the phrase He-Who-Must-Not-Be-Named to describe Lord Voldemort. In general, hyphenated compound words are midway on the journey between being rendered as separate words to being rendered as one word.

A closed compound word is a word that is made up of two words joined together without hyphens or spaces. This type of compound is also called a solid compound word. With the advent of the internet and a more relaxed attitude toward spelling, many compound words have quickly gone from hyphenated compound words to closed compound words. An excellent example of this is the word email. When the phenomenon of email first became available to the general public, the prevalent term was electronic mail. Eventually, this term was shortened to E-mail, with a capital E standing in for the word electronic. Not long after, the capital E evolved to the lowercase, as in e-mail. This spelling is still acceptable according to the Oxford English Dictionary, though it is a secondary spelling. Today, the most accepted form of the word email is the closed compound.

Compound words are usually coined to explain a new phenomenon, idea or invention. The more often a term is used, the faster its journey from two separate words to hyphenation, and finally, one word.

Words in transition