A compound is a word (lexeme) that consists of more than one free morpheme.

A certain type of compounds (endocentric) consist of a head, i.e. the categorical part that contains the basic meaning of the whole compound, and modifiers, which restrict this meaning. For example, the English compound doghouse, where house is the head and dog is the modifier, is understood as a house intended for a dog. Obviously, an endocentric compound tends to be of the same part of speech (word class) as its head.

In other cases, the compound does not have a head, and its meaning cannot be transparently guessed from its constituent parts. For example, the English compound white-collar is neither a kind of collar nor a white thing. In the Sanskrit tradition, this is called a bahuvrihi compound; another (modern) term is exocentric compound, meaning that the concept represented by the compound lies outside its parts. In an exocentric compound, the word class is determined lexically, disregarding the class of the constituents. For example, a must-have is not a verb but a noun.

Composition should not be confused with derivation, where bound morphemes are added to free ones.

A special kind of composition is incorporation, of which noun incorporation into a verbal root (as in English backstabbing, breastfeed, etc.) is most prevalent (see below).

Formation of compounds

Compound formation rules vary widely across language types.

In a perfectly analytic language, compounds are simply elements strung together without any markers. In English, for example, science fiction is a compound noun that consists of two nouns and no markers. A corresponding example from the Mandarin language would be Hànyǔ (漢語; simplified: 汉语), or "the Han Chinese language", which also consists of two nouns and no markers.

In a more synthetic language, the relationship between the elements of a compound may be marked. In German, for example, the compound Kapitänspatent consists of the lexemes Kapitän (sea captain) and Patent (license) joined by the genitive case marker -s. In the Latin language, the lexeme paterfamilias contains the (archaic) genitive form familias of the lexeme familia (father).

Agglutinative languages tend to create very long words with derivational morphemes. Compounds may or may not require the use of derivational morphemes also. The well-known Japanese compound 神風 kamikaze consists only of the nouns kami ("god, spirit") and kaze ("wind"). The longest compounds in the world may be found in Finnish and Germanic languages, such as Swedish. German examples include Kontaktlinsenverträglichkeitstest ("contact-lens compatibility test") and the jocular Rheindampfschiffahrtsgesellschaftskapitänsstellvertreter ("Rhine steamship-company vice-captain"). In theory, while these and even longer compounds are possible, they are not used in practice.

Compound types in different languages

Compound nouns

Most natural languages have compound nouns and sometimes compound adjectives. The position of the head within a compound often depends on the branching tendency of the language, i. e. the most common order of constituents in phrases where nouns are modified by adjectives, by possessors, by other nouns, etc. While Germanic languages, for example, are left-branching when it comes to noun phrases (the modifiers come before the head), the Romance languages are usually right-branching.

In French, compound nouns are often formed by left-hand heads with prepositional components inserted before the modifier, as in chemin-de-fer ("railway", lit. "road of iron") and moulin à vent ("windmill", lit. "mill (that works)-by-means-of wind").

Verb-noun compounds

In Spanish there is a very common type of compound noun consisting of a verb (conjugated for third person singular, present tense, indicative mood) followed by a noun (usually plural), such as rascacielos (modelled on "skyscraper", lit. "scratches skies") and sacacorchos ("corkscrew", lit. "removes corks"). These compounds are formally invariable in the plural (but in many cases they have been reanalyzed as plural forms, and a singular form has appeared). French and Italian have these same compounds with the noun in the singular form: Italian grattacielo ("skyscraper"), French grille-pain ("toaster", lit. "toasts bread").

English prefers another type of verb-noun compounds, in which an argument of the verb is incorporated into the verb, which is then usually turned into a gerund, such as breastfeeding, finger-pointing, etc. The noun is usually an instrumental complement.

Compound adpositions

Compound prepositions formed by prepositions and nouns are common in English and the Romance languages (consider English on top of, Spanish encima de, etc.). Japanese shows the same pattern, except the word order is the opposite (with postpositions): no naka ni (lit. "of inside on", i. e. "on the inside of").

Other examples

Spanish:

• Ciencia-ficción ("science fiction"): ciencia, "science", + ficción, "fiction" (This word is a calque from the English expression science fiction. In English, the head of a compound word is the last morpheme: science fiction. Conversely, the Spanish head is located at the front, so ciencia ficción sounds like a kind of fictional science rather than scientific fiction.)
• Ciempiés ("centipede"): cien, "hundred", + pies, "feet"
• Ferrocarril ("railway"): ferro, "iron", + carril, "lane"

Italian:

• Centopiedo ("centipede"): cento, "hundred", + piedi, "feet"
• Ferrovia ("railway"): ferro, "iron", + via, "way"
• Lavacristallo ("windscreen"): lavare, "to wash", + cristallo, "crystal, (pane of) glass"

German:

• Wolkenkratzer ("skyscraper"): wolken, "clouds", + kratzer, "scraper"
• Eisenbahn ("railway"): Eisen, "iron", + bahn, "track"
• Kraftfahrzeug ("automobile"): Kraft, "power", + fahren/fahr, "drive", + zeug. "machinery"
• Stacheldraht ("barbed wire"): stachel, "barb/barbed", + draht, "wire"

See also

• Bahuvrihi compounds
• English compounds
• Portmanteau compounds
• Sanskrit compounds
• Incorporation (linguistics)