Pied-piping and prepositional stranding
Pied-piping describes the situation where a phrase larger than a single wh-word occurs in the fronted position. In the case where the wh-word is a determiner such as which or whose, pied-piping refers to the wh-determiner's appearance sentence-initially along with its complement. For instance, in the following example, the entire phrase "which car" is moved:
Which car does he like t?
In the transformational analysis, the wh-word which moves to the beginning of the sentence, taking car, its complement, with it, much as the Pied Piper of Hamelin attracted rats and children to follow him, hence the term pied-piping.
In the case of determiners, pied-piping is obligatory. For instance, the following sentence would be ungrammatical:
*Which does he like t car?
However, there are cases where pied-piping can be optional. In English, this is often the case when a wh-word or phrase is the object of a preposition. For instance, the following two examples are both grammatical:
To whom did she reveal her secret t?
Who did she reveal her secret to t?
The second example is a case of preposition stranding, which is possible in English, but not allowed in Latin or other Romance languages. For languages that use postpositions rather than prepositions, stranding is not allowed either.