In linguistics, pied-piping is a phenomenon of syntax whereby a given focused expression takes an entire encompassing phrase with it when it is "moved". The term itself is due to John Robert Ross; it is a reference to the Pied Piper of Hamelin, the figure of fairy tales who lured rats (and children) by playing his flute. Pied-piping is an aspect of discontinuities in syntax, having to do with the constituents that can and cannot be discontinuous. While pied-piping is most visible in cases of wh-fronting of information questions and relative clauses, it is not limited to wh-fronting, but rather it can be construed as occurring with most any type of discontinuity (extraposition, scrambling, topicalization). Most if not all languages that allow discontinuities employ pied-piping to some extent, although there are major differences across languages in this area, some languages employing pied-piping much more than others.
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Examples
Typical examples of pied-piping occur when a wh-expression drags with it an entire encompassing phrase to the front of the clause. The focused expression is in bold in the following examples and the fronted word/phrase in the b- and c-sentences is underlined. The material that has been pied-piped is therefore any underlined material that is not bolded. The gap marks the canonical position of the fronted expression:
In each of the b-sentences, the interrogative word has pied-piped an encompassing phrase with it, whereas each c-sentence is bad because pied-piping has not occurred. These examples illustrate that the pied-piped phrase can be a noun phrase, an adjective phrase, or an adverb phrase, and examples further below illustrate that it can also be a prepositional phrase. Pied-piping occurs in embedded wh-clauses as well:
And pied-piping is very frequent in relative clauses, where a greater flexibility about what can or must be pied-piped is discernible:
Wh-clauses vs. relative clauses
The pied-piping mechanism is more flexible in relative clauses in English than in interrogative clauses, because it can pied-pipe material that would be less acceptable in the corresponding interrogative clause, e.g.
The d-examples, where pied-piping has occurred in a relative clause, are acceptable, whereas the corresponding wh-clauses in the b- and c-sentences are much less acceptable. This aspect of pied-piping - i.e. that it is more restricted in wh-clauses than in relative clauses in English - is poorly understood, especially in light of the fact that the same contrast in acceptability does not obtain in closely related languages such as German.
Preposition stranding
Pied-piping in English is optional with many prepositions (in, of, on, to, with, etc.). Colloquial registers prefer to avoid pied-piping whenever possible, whereas a more formal register can opt for pied-piping. The issue is understood in terms of preposition stranding. When a preposition is stranded, pied-piping has not occurred, whereas when preposition stranding is avoided, pied-piping of the preposition has occurred, e.g.
The debate surrounding the stylistic acceptability of preposition stranding can hence be illuminated by an understanding of pied-piping.
Pied-piping broadly construed
Broadly construed, pied-piping occurs in other types of discontinuities beyond wh-fronting, that is, if one views just part of a topicalized or extraposed phrase as focused, then pied-piping can be construed as occurring with these other types of discontinuities, e.g.
Assuming that just the bolded words in these examples bear contrastive focus, the rest of the topicalized or extraposed phrase is pied-piped in each b-sentence. Similar examples could be produced for scrambling.
Pied-piping across languages
Pied-piping varies significantly across languages. Languages with relatively strict word order tend to employ pied-piping more often than languages that have freer word order. Hence English, with its relatively strict word order, employs pied piping more often than, for instance, the Slavic languages, with their relatively free word order. Three examples - one from Russian, one from Latin, and one from German - are now employed to illustrate the variation in pied-piping across languages. Unlike in English, a pre-noun modifier in Russian and Latin need not pied-pipe the noun that it modifies, e.g.
When the word order here of Russian and Latin is maintained in English, the sentences are bad. They are bad because pre-noun modifiers necessarily pied-pipe their noun in English. The constraint is known as the Left Branch Condition: a modifier on a left branch under a noun may not be extracted from the noun phrase. Apparently, the Left Branch Condition is absent from Russian. The second example illustrating variation in pied-piping across languages is from German. Relative pronouns in German at times have the option to pied-pipe a governing zu-infinitive when they are fronted, e.g.
The a-sentence has the relative pronoun das pied-piping the zu-infinitive zu lesen to the front of the relative clause, whereas this pied-piping does not occur in the b-sentence. Since both variants are acceptable, pied-piping in such cases is optional. In English in contrast, pied-piping in such constellations is impossible, e.g. *the book which to read I tried and *the book to read which I tried.
The fact that pied-piping varies so much across languages is a major challenge facing theories of syntax.