When you divide up a complex word into its morphemes, it’s easy to get the impression that words are put together like the beads that make up a necklace – one after the other in a line:

(11)unhappiness =un +happy+ ness

But morphologists believe that words are more like onions than like necklaces: onions are made up of layers from innermost to outermost. Consider a word like unhappiness. We can break this down into its component morphemes un +happy +ness, but given what we learned above about the properties of the prefix un- and the suffix -ness we know something more about the way in which this word is constructed beyond just its constituent parts. We know that un- must first go on the base happy. Happy is an adjective, and un- attaches to adjectives but does not change their category. The suffix -ness attaches only to adjectives and makes them into nouns. So if un- attaches first to happy and -ness attaches next, the requirements of both affixes are met. But if we were to do it the other way around, -ness would have first created a noun, and then un- would be unable to attach. We could represent the order of attachment as if words really were onions, with the base in the innermost layer, and each affix in its own succeeding layer: see figure 3.2.

But linguists, not generally being particularly artistic, prefer to show these relationships as ‘trees’ that look like this:

Similarly, we might represent the structure of a word like repurify as in :

In order to draw this structure, we must first know that the prefix reattaches to verbs (for example, reheat, rewash, or redo) but not to adjectives (*repure, *rehappy) or to nouns (*rechair, *retruth). Once we know this, we can say that the adjective pure must first be made into a verb by suffixing -ify, and only then can re- attach to it.