Kinds of morpheme: bound versus free
The morphemes in the word helpfulness, just discussed, do not all have the same status. Help, -ful and -ness are not simply strung together like beads on a string. Rather, the core, or starting-point, for the formation of this word is help; the morpheme -ful is then added to form helpful, which in turn is the basis for the formation of helpfulness.
There are two reasons for calling help the core of this word. One is that help supplies the most precise and concrete element in its meaning, shared by a family of related words like helper, helpless, helplessness and unhelpful that differ from one another in more abstract ways. Another reason is that, of the three morphemes in helpfulness, only help can stand on its own – that is, only help can, in an appropriate context, constitute an utterance by itself. That is clearly not true of -ness, nor is it true of -ful. (Historically -ful is indeed related to the word full, but their divergence in modern English is evident if one compares words like helpful and cheerful with other words that really do contain full, such as half-full and chock-full.) In self-explanatory fashion, morphemes that can stand on their own are called free, and ones that cannot are bound.
A salient characteristic of English – a respect in which English differs from many other languages – is that a high proportion of complex words are like helpfulness and un-Clintonish in that they have a free morpheme (like help and Clinton) at their core. Compare the two column of words listed at (1), all of which consist uncontroversially of two morphemes, separated by a hyphen:
(1) a. read-able b. leg-ible
The rationale for the division is that the words in column a. all contain a free morpheme, respectively read, hear, large, perform, white and dark. By contrast, in the words in column b., though they are similar in meaning to their counterparts in a., both the morphemes are bound. If you know something about the history of the English language, or if you know some French, Spanish or Latin, you may know already that most of the free morphemes in (1a) belong to that part of the vocabulary of English that has been inherited directly through the Germanic branch of the Indo-European language family to which English belongs, whereas all the morphemes in (1b) have been introduced, or borrowed, from Latin, either directly or via French. Even without such historical knowledge, it may strike you that the words in (1b) are on the whole somewhat less common, or more bookish, than those in (1a). This reflects the fact that, among the most widely used words, the Germanic element still predominates. It is thus fair to say that, in English, there is still a strong tendency for complex words to contain a free morpheme at their core. Is it possible for a bound morpheme to be so limited in its distribution that it occurs in just one complex word? The answer is yes. This is almost true, for example, of the morpheme leg- ‘read’ in legible at (1b): at least in everyday vocabulary, it is found in only one other word, namely illegible, the negative counterpart of legible. And it is absolutely true of the morphemes cran-, huckle- and gorm- in cranberry, huckleberry and gormless. Cranberry and huckleberry are compounds whose second element is clearly the free morpheme berry, occurring in several other compounds such as strawberry, blackberry and blueberry; however, cran- and huckle- occur nowhere outside these compounds. A name commonly given to such bound morphemes is cranberry morpheme. Cranberry morphemes are more than just a curiosity, because they reinforce the difficulty of tying morphemes tightly to meaning. What does cran- mean? Arguably, nothing at all; it is only the entire word cranberry that can be said to be meaningful, and it is certainly the entire word, not cran- by itself, that is in any dictionary. (You may have noticed, too, that although blackberries are indeed blackish, strawberries have nothing obvious to do with straw; so, even if straw- in strawberry is not a cranberry morpheme, it does not by itself make any predictable semantic contribution in this word.)
Carstairs, Andrew and McCarthy (2002), An Introduction to English Morphology: Words and Their Structure, Edinburgh: Edinburgh University Press Ltd.
Lieber, Rochelle (1992), Deconstructing Morphology: Word Formation in Syntactic Theory, Chicago: University of Chicago Press.