By desmond bett; B. A- criminology m. A – public administration & policy

3.3.2
Contrast
Different morpheme CONTRAST meanings but different allomorphs do not. If a difference in meaning is attributable to the fact that one minimal meaningful unit has been replaced by another, we identify the morphs involved as manifestations of distinct morphemes. So, in [3.7] on p. 36 the indefinite article realized by a and an is a distinct morpheme from the definite article realized by the since a semantic difference is detectable when a and an is replaced with the.
A further example of contrast is given in [3.10]:
[3.10]:
a. I unlocked the door. b. She is untidy.
I re-locked the door
The two sentences in [3.10a] mean very different things. Since they are identical except for the fact that where one has un- the other has re-, the difference in meaning between these two sentences is due to the difference in meaning between the morphemes realized by re-, (meaning ‘do again’) an un- (meaning ‘reverse the action’).
Now, contrast the un- of unlocked with the un- of untidy. In both cases we have the same morph un- (which is spelt and pronounced in exactly the same way). But it is obvious that un- represents different morphemes in these two word-forms. In I unlocked the door the morphs un- found in unlocked realizes a negative morpheme attached to verbs-it reverses the action of locking. But in untidy it realizes a negative morpheme attached to adjectives- untidy means ‘not tidy’. (If a person is untidy, it does not mean that at some earlier point they were tidy and someone has reverse or undone their tidiness.)
If morphemes were made up of phonemes a simple correlation of morphs with morphemes is what we would find. But, in fact, it is quite common for the same phonological form (i.e. morph) to represent more than one morpheme. It is from the context that we can tell which morpheme it represents. This is the second piece of evidence against the assumption that morphemes are composed of phonemes.
The complex relationship between morphemes and the allomorphs that represent them gives us a window through which we can glimpse one of the most fascinating aspects of language structure; the relationship between FORM and FUNCTION. In linguistics we explore the form of various elements of language structure, e.g. words and sentences, because it is important to know how they are constructed. However, form is not everything. We are also interested in knowing what linguistic elements are used so far, what function they serve.
Just consider for a moment this non-linguistic analogy. Imagine a friend from a foreign vacation with two beautiful ornament glass containers with a globular shape and gives one to you as a present and keeps the other for herself. She does not tell you what your present is used for. She uses hers as a vessel for containing wine at the table –she got the idea of buying these containers when she was served wine in a similar container in a fancy restaurant. You do not know this. You look at your present and decide to put it on the table as a container for cut fresh flowers. She calls her a flagon, for that is what she is using it as. You call yours a vase.
Here are the questions now: are these objects ‘flagons’ or ‘vase’? Which one of you is right? I am not being evasive if I say that both of you are right. For, although the two objects are identical as far as their form, their physical properties, is concerned. They are very different with regard to the functions that they serve in your two households.
There are numerous linguistic parallel. What is physically the same linguistic form can be used to represent distinct morpheme. In order for forms to be regarded as allomorphs belonging to the same morpheme. It is not sufficient for them to have the same form-to be pronounced or written in the same way. They must also have the same grammatical or semantic function. The significance of this point was hinted at the discussion of un- in unlocked and untidy when we showed that the same morph can represent different morphemes. It should become even more obvious when you consider the form –er in the following:
[3.11]
a. think - thinker drive- driver
write - writer sing- singer
sweep – sweeper sell – seller
b. cook – cooker strain – strainer
receive – receiver compute – computer
propel – propeller erase – eraser
c. London – Londoner north – northerner
Iceland – Icelander east – easterner
New York – New Yorker Highlands - Highlander

The same form, -er, represents three different meanings and hence has to be assigned to three distinct morphemes. In [3.11a] it forms an agentive noun from a verb, with the meaning ‘someone who does X’ (i.e. whatever the verb means). In [3.11b] the same –er forms an instrumental noun from a verb, with the meaning ‘something use to X’ (i.e. whatever the verb means). Finally, in [3.11c] the same –er is attached to a noun referring to a place to mean ‘an inhabitant of’.

Clearly, the sane form does serve different functions here. So, it realizes different morphemes. This is further evidence that shout quickly disabuse us of the assumption that morphemes. Are made up of morphs. Not only can a single morpheme has several allomorphs (as in the case of the plural morpheme), the same morph (e.g. –er) can represent morphemes. There is no simple one-to-one matching of morphemes with morphs.