In morpheme-based morphology, a null allomorph or zero allomorph is a special kind of allomorph which has the form of a null morpheme. Because there are contexts in which the underlying morpheme still appears in the surface structure, a "real" null morpheme could be re-defined as a morpheme which never occurs in the surface representation of words. The phenomenon itself is known as null allomorphy or total morpheme blocking. 
An example of null allomorphy in English is the phrase two fish-Ø which can also be two fish-es. In addition, the forms of many auxiliary verbs such as do may have null allomorphs, especially in children's language.
Null allomorphy occurs a lot in the grammar of the German language. The singular form of the dative case of masculine and neuter nouns such as der Mann (the man) has an optional grammatical suffix -e: dem Mann-e. However, this suffix is somewhat archaic today and is mainly used in written language. In other cases, its null allomorph occurs: dem Mann-Ø.
Null allomorphs also occur in the Dutch language, be it to a lesser degree than in German. That is, many Dutch compound words have an interfix -s, the use of which is completely optional: Dutch people both say doodkist and doodskist ("coffin"), and both spellingprobleem and spellingsprobleem ("spelling problem").
- ^ http://184.108.40.206/dissertations/Pycha_dissertation_2008.pdf
- ^ http://books.google.com/books?id=jTZ5Fq6DJSQC&pg=PA146&lpg=PA146&dq=%22null+allomorph%22&source=bl&ots=PCbu_xLSuI&sig=biTtfkqwPyyQbLIMwBNZaQ1uxiA&hl=nl&ei=6lHOSsTEN86Q-Abc1b2KAw&sa=X&oi=book_result&ct=result&resnum=6#v=onepage&q=%22null%20allomorph%22&f=fals
- ^ (Dutch)
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