The most frequently used example involves embedding a relative clause inside another one as in: In theories of natural language parsing, the difficulty with multiple center embedding is thought to arise from limitations of the human short term memory.
An interesting theoretical point is that sentences with multiple center embedding are grammatical, but unacceptable.
"[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?"
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).
In spoken language, multiple center-embeddings even of degree 2 are so rare as to be practically non-existing.