We showed how, in English derivational morphology, suffixes heavily outnumber prefixes. We saw that most compounds are headed, with the head on the right. Superficially these two facts are unconnected. Consider, however, the role played by the head house of a compound such as greenhouse. As head, house determines the compound’s syntactic status (as a noun), and also its meaning, inasmuch as a greenhouse is a kind of house for plants. This is very like the role played by the suffix -er in the derived word teacher: it determines that teacher is a noun, unlike its base, the verb teach, and it contributes the meaning ‘someone who Xs’, where the semantic blank X is here filled in by teach. Many (though not all) linguists therefore treat -er as the head of teacher in just the same way as house is the head of greenhouse. This is relevant to the distinction between helpful and *-ful-help. In helpful, the affix is what determines that the whole word is an adjective, and so counts as its head. Accordingly, *-ful-help violates English expectations not just because the affix is on the wrong side, but also because the rightmost element is not the head. In the derived words teacher and helpful, therefore, the two components are not equal contributors, so to speak; rather, the righthand element (as in most compounds) has a special status.

Superficially, this view of affixes as heads leads us to expect that prefixed words should be as rare in English as left-headed compounds are (items such as attorney general). Yet prefixes, though fewer than suffixes, include some that are of very common occurrence, such as un- ‘not’ and re- ‘again’. Is our expectation disappointed, then? Not really, despite first appearances. Consider the relationship between helpful and unhelpful. In helpful, -ful has a clearly wordclass-determining role because it changes a noun, help, into an adjective. In unhelpful, however, un- has no such role; rather, it leaves the wordclass of helpful unchanged. This characteristic of un- is not restricted to adjectives, moreover. Verbs to which un- is prefixed remain verbs (e.g. untie, unfasten, unclasp), and those few nouns to which un- is prefixed remain nouns (unease, unrest). This strongly suggests that the head of of all these words is not un- but the base to which un- is attached (helpful, tie, ease etc.) – and which is the righthand element.

Similar arguments apply to re-: rearrange, repaint and re-educate are verbs, just as arrange, paint and educate are. These prefixed verbs, therefore, are right-headed also. The only prefixes that are unequivocally heads are those that change wordclass, such as de- in delouse (deriving verbs from nouns) and en- in enfeeble and enslave (deriving verbs from nouns and adjectives). So, while left-headed derived words do exist, just as left-headed compounds do, they are also not so numerous as may at first appear.