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 (karlsson 2007).