It is with nouns that compounding really comes into its own as a word forming process in English. That is not surprising. Cultural and technical change produces more novel artefacts than novel activities or novel properties. These changes therefore generate new vocabulary needs that are more readily answered by new nouns than by new verbs or adjectives. Examples can be found with each of the other main word classes supplying the left-hand element:

(13) verb–noun (VN): swearword, drophammer, playtime

(14) noun–noun (NN): hairnet, mosquito net, butterfly net, hair restorer

(15) adjective–noun (AN): blackboard, greenstone, faintheart

(16) preposition–noun (PN): in-group, outpost, overcoat

All of these have the main stress on the left – a characteristic identified as important for distinguishing compound nouns from noun phrases. (The fact that hair restorer, butterfly net and mosquito net are spelled with a space does not affect the fact that, from the grammatical point of view, they each constitute one complex word.) Most of these are also right-headed, although we will defer further discussion of headedness.

If you try to think of more examples for the four types at (13)–(16), you will probably find the task easiest for the NN type at (14). In fact, almost any pair of nouns can be juxtaposed in English so as to form a compound or a phrase – provided that there is something that this compound or phrase could plausibly mean. The issue of meaning turns out to play an important part in distinguishing two kinds of NN compound. Consider the four examples at (14). Does each one have a precise interpretation that is clearly the most natural, on the basis of the meanings of their two components? For hair restorer, the answer is surely yes: it most naturally denotes a substance for restoring hair growth. On the other hand, for hairnet, butterfly net and mosquito net the answer is less clear. What tells us that a hairnet is for keeping one’s hair in place, while a butterfly net is for catching butterflies and a mosquito net is for keeping mosquitoes away? This information does not reside in the meaning of net, nor in the meanings of hair, butterfly and mosquito. The most that one can conclude from these individual meanings is that each is a net that has something to do with hair, butterflies and mosquitoes respectively. Arriving at the precise meanings of these compounds depends on our knowledge of the world (that some people collect butterflies, and that mosquitoes can carry disease) rather than on purely linguistic knowledge.

The difference in precision with which we can interpret hair restorer on the one hand and hairnet etc. on the other hinges on the fact that restorer in hair restorer is derived from a verb (restore). Verbs,unlike most nouns and adjectives, impose expectations and requirements on the noun phrases that accompany them in the sentence. For example, with the verb sleep we expect to find one noun phrase as subject; with eat we expect to find also a noun phrase as object; and with give we expect to find, or at least to be able to identify from the context, a third ‘indirect object’ noun phrase denoting the recipient of the gift. These expected or required nominal concomitants to a verb are called its arguments. For present purposes, what matters is that, when the head of a NN compound is derived from a verb, as restorer is, the most natural way to interpret the whole compound is quite precise: the first element expresses the object argument of the verb (that is, the person or thing that undergoes the action). For example, an X-restorer, whatever X is, something or someone that restores X.

Here are some more compounds whose second element is derived from a verb:

(17) sign-writer, slum clearance, crime prevention, wish-fulfilment

For all of these, the most natural interpretation is clear. To interpret any of them some other way – for example, to interpret crime prevention as meaning not ‘prevention of crime’ but ‘use of crime for preventive purposes’ – seems contrived and unnatural.

It is time to introduce some terminology, for convenience. Let us call a NN compound like hairnet or mosquito net, in which the right-hand noun is not derived from a verb and whose interpretation is therefore not precisely predictable on a purely linguistic basis, a primary or root compound. (The term ‘root compound’ is well established but not particularly appropriate, because primary compounds include many, such as climbing equipment or fitness campaigner, neither of whose components is a root) Let us call a NN compound like hair restorer or slum clearance, in which the first element is interpreted as the object of the verb contained within the second, a secondary or verbal compound. (Yet another term sometime used is synthetic compound.) Paradoxically, then, although verbs are relatively rare as elements in compounds in English (the swearword pattern is unusual), verbal compounds, in the sense just defined, are common.

Secondary compounds are certainly right-headed, in that (for example) crime prevention denotes a kind of prevention and wish-fulfilment denotes a kind of fulfilment. In this respect they are like most NN compounds and most compounds generally – but not all.