A compound noun is a noun phrase made up of two nouns, e.g. bus driver, in which the first noun acts as a sort of adjective for the second one, but without really describing it. (For example, think about the difference between a black bird and a blackbird.)

Figure 1. A crow is a black bird, while a blackbird is a specific species of bird.

Compound nouns can be made up of two or more other words, but each compound has a single meaning. They may or may not be hyphenated, and they may be written with a space between words—especially if one of the words has more than one syllable, as in living room. In that regard, it’s necessary to avoid the over-simplification of saying that two single-syllable words are written together as one word. Thus, tablecloth but table mat, wine glass but wineglassful or key ring but keyholder. Moreover, there are cases which some people/dictionaries will write one way while others write them another way. Until very recently we wrote (the) week’s end, which later became week-end and then our beloved weekend.

Types of Compound Nouns

Short compounds may be written in three different ways:

• The solid or closed forms in which two usually moderately short words appear together as one. Solid compounds most likely consist of short units that often have been established in the language for a long time. Examples are housewife, lawsuit, wallpaper, basketball, etc.
• The hyphenated form in which two or more words are connected by a hyphen. This category includes compounds that contain suffixes, such as house-build(er) and single-mind(ed)(ness). Compounds that contain articles, prepositions or conjunctions, such as rent-a-cop and mother-of-pearl, are also often hyphenated.
• The open or spaced form consisting of newer combinations of usually longer words, such as distance learning, player piano, lawn tennis, etc.

Hyphens are often considered a squishy part on language (we’ll discuss this further in Hyphens and Dashes). Because of this, usage differs and often depends on the individual choice of the writer rather than on a hard-and-fast rule. This means open, hyphenated, and closed forms may be encountered for the same compound noun, such as the triplets container ship/container-ship/containership and particle board/particle-board/particleboard. If you’re ever in doubt whether a compound should be closed, hyphenated, or open, dictionaries are your best reference.

Plurals

The process of making compound nouns plural has its own set of conventions to follow. In all forms of compound nouns, we pluralize the chief element of a compound word (i.e., we pluralize the primary noun of the compound).

• fisherman → fishermen
• black bird → black birds
• brother-in-law → brothers-in-law

The word hand-me-down doesn’t have a distinct primary noun, so its plural is hand-me-downs.