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The Types of Morphemes
Morphemes can be classified from different view-points:

1. 
   functional
   
2. 
   number correlation between form and content
   

From the point of view of function they may be lexical and grammatical. The lexical morphemes are those that express full lexical meaning of their own and are associated with some object, quality, action, number of reality, like: lip, red, go, one and so on. The lexical morphemes can be subdivided into lexical - free and lexical - bound morphemes. The examples given above are free ones; they are used in speech independently. The lexical-bound ones are never used independently; they are usually added to some lexical-free morphemes to build new words like- friend-ship, free-dom, teach-er, spoon-ful and so on. Taking into account that in form they resemble the grammatical inflections they may be also called lexical - grammatical morphemes. Thus lexical - bound morphemes are those that determine lexical meanings of words but resemble grammatical morphemes in their dependence on lexical - free morphemes. The lexical - bound morphemes are means to build new words.
The grammatical morphemes are those that are used either to connect words in sentences or to form new grammatical forms of words. The content of such morphemes are connected with the world of reality only indirectly therefore they are also called structural morphemes, e.g., shall, will, be, have, is, -(e)s, -(e)d and so on. As it is seen from the examples the grammatical morphemes have also two subtypes: grammatical - free and grammatical - bound. The grammatical - free ones are used in sentences independently (I shall go) while grammatical - bound ones are usually attached to some lexical - free morphemes to express new grammatical form, like: girl's bag, bigger room, asked.
From the point of view of number correlation between form and content there may be overt, zero, empty and discontinuous morphemes.
By overt morpheme the linguists understand morphemes that are represented by both form and content like: eye, bell, big and so on.
Zero morphemes are those that have (meaning) content but do not have explicitly expressed forms. These morphemes are revealed by means of comparison:
ask – asks high -higher
In these words the second forms are marked: "asks" is a verb in the third person singular which is expressed by the inflection "s". In its counterpart there's no marker like "s" but the absence of the marker also has grammatical meaning: it means that the verb "ask" is not in the third person, singular number. Such morphemes are called "zero". In the second example the adjective "higher" is in the comparative degree, because of the "- er" while its counterpart "high" is in the positive degree, the absence of the marker expresses a grammatical meaning, i.e. a zero marker is also meaningful, therefore it's a zero morpheme.
There are cases when there's a marker which has not a concrete meaning, i.e. there's neither lexical nor grammatical meaning like: statesman. The word consists of three morphemes: state - s - man. The first and third morphemes have certain meanings. But "s" has no meaning though serve as a connector: it links the first morpheme with the third one. Such morphemes are called empty. Thus empty morphemes are those that have form but no content.
In contemporary English there are cases when two forms express one meaning like:
He is writing a letter
Two morphemes in this sentence "is" and " - ing" express one meaning: a continuous action. Such morphemes are called discontinuous.
Thus there are two approaches to classify morphemes: functional and number correlation between form and content.
The first one can be shown in the following scheme:
Morphemes
lexical grammatical
free bound free bound
The second one can also be shown in the same way:
Morphemes

	overt	zero	empty	discontinuous
form	+	-	+	++
meaning	+	+	-	+