Center embedding

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In linguistics, center embedding is the process of embedding a phrase in the middle of another phrase of the same type. This often leads to difficulty with parsing which would be difficult to explain on grammatical grounds alone. The most frequently used example involves embedding a relative clause inside another one as in:

A man that a woman loves ${\displaystyle \Rightarrow }$

A man that a woman that a child knows loves ${\displaystyle \Rightarrow }$

A man that a woman that a child that a bird saw knows loves ${\displaystyle \Rightarrow }$

A man that a woman that a child that a bird that I heard saw knows loves

In theories of natural language parsing, the difficulty with multiple center embedding is thought to arise from limitations of the human short term memory. In order to process multiple center embeddings, we have to store many subjects in order to connect them to their predicates.

An interesting theoretical point is that sentences with multiple center embedding are grammatical, but unacceptable. Such examples are behind Noam Chomsky's comment that, "Languages are not 'designed for parsability' … we may say that languages, as such, are not usable."^{[citation needed]}

Some researchers (such as Peter Reich) came up with theories that though single center embedding is acceptable (as in "the man that boy kicked is a friend of mine"), double center embedding is not. The linguist Anne De Roeck and colleagues provided a counter-example: "Isn't it true that example-sentences that people that you know produce are more likely to be accepted?" (De Roeck et al., 1982).

The linguist Fred Karlsson provided empirical evidence in 2007 that the maximal degree of multiple center-embedding of clauses is exactly 3 in written language. He provided thirteen genuine examples of this type from various Indo-European languages (Danish, English, German, Latin, Swedish). No real examples of degree 4 have been recorded. In spoken language, multiple center-embeddings even of degree 2 are so rare as to be practically non-existing.^[1]

Center embedding is the focus of a science fiction novel, Ian Watson's The Embedding, and plays a part in Ted Chiang's Story of Your Life.