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In linguistics, a morpheme is the smallest grammatical unit in a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word, by definition, is freestanding. When it stands by itself, it is considered a root because it has a meaning of its own (e.g. the morpheme cat) and when it depends on another morpheme to express an idea, it is an affix because it has a grammatical function (e.g. the –s in cats to specify that it is plural). Every word comprises one or more morphemes. The more combinations a morpheme is found in, the more productive it is said to be.
Classification of morphemes
Free vs. bound
Every morpheme can be classified as either free or bound. These categories are mutually exclusive, and as such, a given morpheme will belong to exactly one of them.
- Free morphemes can function independently as words (e.g. town, dog) and can appear with other lexemes (e.g. town hall, doghouse).
- Bound morphemes appear only as parts of words, always in conjunction with a root and sometimes with other bound morphemes. For example, un- appears only accompanied by other morphemes to form a word. Most bound morphemes in English are affixes, particularly prefixes and suffixes, examples of suffixes are: tion, ation, ible, ing, etc. Bound morphemes that are not affixes are called cranberry morphemes.
Bound morphemes can be further classified as derivational or inflectional.
- Derivational morphemes, when combined with a root, change either the semantic meaning or part of speech of the affected word. For example, in the word happiness, the addition of the bound morpheme -ness to the root happy changes the word from an adjective (happy) to a noun (happiness). In the word unkind, un- functions as a derivational morpheme, for it inverts the meaning of the word formed by the root kind.
- Inflectional morphemes modify a verb's tense or a noun's number without affecting the word's meaning or class. Examples of applying inflectional morphemes to words are adding -s to the root dog to form dogs and adding -ed to wait to form waited. In English, there are eight inflections.
Allomorphs are variants of a morpheme that differ in pronunciation but are semantically identical. For example, in English, the plural marker -(e)s of regular nouns can be pronounced /-z/, /-s/, or /-ɨz/, depending on the final sound of the noun's singular form.
Content vs. function
Content morphemes express a concrete meaning or content, while function morphemes have more of a grammatical role. For example, the morphemes fast and sad can be considered content morphemes. On the other hand, the suffix –ed belongs to the function morphemes given that it has the grammatical function of indicating past tense. Although these categories seem very clear and counterintuitive, the idea behind it can be harder to grasp given that they overlap with each other. Examples of an ambiguous situation are the preposition over and the determiner your, which seem to have a concrete meaning, but are considered function morphemes because their role is to connect ideas grammatically. A general rule to follow to determine the category of a morpheme is:
- Content morphemes include free morphemes that are nouns, adverbs, adjective, and verbs. It also includes bound morphemes that are bound roots and derivational affixes. 
- Function morphemes can be free morphemes that are prepositions, pronouns, determiners, and conjunctions. Additionally, they can be bound morphemes that are inflectional affixes. 
First, roots are composed of only one morpheme while stems can be composed of more than one morpheme. Also, any additional affixes are considered morphemes. An example of this is the word quirkiness. The root is quirk, but the stem is quirky which has two morphemes. Second, another thing to take in consideration is that there might be affixes that have the same phonological form, but have different meaning. For example, the suffix –er can be derivative (e.g. wonder ⇒ wonderer) or inflectional (e.g. small ⇒ smaller). These types of morphemes are called homophonous.
A final factor to keep in consideration is to not be confused by monomorphemic words, which contain only one morpheme. For instance, some words might seem to be composed of multiple morphemes, but it fact they are not. This is why we have to consider form and meaning when searching for morphemes. For example, we might think that the word relate is composed of two morphemes, re- (prefix) and the word late, but this is not correct. It has no relationship with the definitions relevant to the word like “feel sympathy”, “narrate”, or “being connected by blood or marriage”. Furthermore, the length of the words does not determine if it has multiple morphemes or not. To demonstrate, the word Madagascar is long and it might seem to have morphemes like mad, gas, and car, but it does not. Conversely, small words can have multiple morphemes (e.g. dogs).
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In natural language processing for Korean, Japanese, Chinese and other languages, morphological analysis is the process of segmenting a sentence into a row of morphemes. Morphological analysis is closely related to part-of-speech tagging, but word segmentation is required for these languages because word boundaries are not indicated by blank spaces.
The purpose of morphological analysis is to determine the minimal units of in a language or morphemes by using comparisons of similar forms. For example, comparing forms such as “She is walking” and “They are walking” rather than comparing any of the previous sentences with something completely different like “You are reading”. Thus, we can effectively break down the forms in parts and distinguishing the different morphemes. Similarly, keep in mind that the meaning and the form are equally important during the identification of morphemes. For instance, agent and comparative morphemes illustrate this point. An agent morpheme is an affix like -er that transforms a verb into a noun (e.g. teach ⇒ teacher). On the other hand, –er can also be a comparative morpheme that changes an adjective into another degree of the same adjective (e.g. small ⇒ smaller). In this case, the form is the same, but the meaning of both morphemes is different. Also, the opposite can occur in which the meaning is the same but the form is different.
Changing definitions of morpheme
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In generative grammar, the definition of a morpheme depends heavily on whether syntactic trees have morphemes as leaves or features as leaves.
- Direct surface to syntax mapping LFG – leaves are words
- Direct syntax to semantics mapping
Given the definition of morpheme as "the smallest meaningful unit" Nanosyntax aims to account for idioms where it is often an entire syntactic tree which contributes "the smallest meaningful unit." An example idiom is "Don't let the cat out of the bag" where the idiom is composed of "let the cat out of the bag" and that might be considered a semantic morpheme, which is composed of many syntactic morphemes. Other cases where the "smallest meaningful unit" is larger than a word include some collocations such as "in view of" and "business intelligence" where the words together have a specific meaning.
The definition of morphemes also play a significant role in the interfaces of generative grammar in the following theoretical constructs;
- Event semantics: the idea that each productive morpheme must have a compositional semantic meaning (a denotation), and if the meaning is there, there must be a morpheme (null or overt).
- Spell-out: the interface where syntactic/semantic structures are "spelled-out" using words or morphemes with phonological content. This can also be thought of as lexical insertion into the syntactics.
- Kemmer, Suzanne. "Words in English: Structure". Retrieved 10 April 2014.
- Packer, Martin. "Morphology". Retrieved 20 March 2014.
- Morphology Classification Of Morphemes Referenced 19 March 2014
- "Morphology II". Retrieved 10 April 2014.
- Department of Linguistics (2011). Language files: Materials for an introduction to language and linguistics (11th ed.). Ohio State University Press.
- Spencer, Andrew (1992). Morphological Theory. Oxford: Blackwell.
|Look up morpheme in Wiktionary, the free dictionary.|
- Glossary of reading terms
- Comprehensive and searchable morpheme reference
- Linguistics 001 — Lecture 7 — Morphology by Prof. Mark Lieberman
- Pronunciation of the word morpheme