English avoids a locution found in many other languages, including daughter languages of English: all what.  We also avoid From where? In a direct or indirect question, but allow from where as a quasi-relative in, e.g., They flew to Honolulu, from where they went to Hilo by sea.  What follows is a consolidation and updating of information published by me in the eighties.  The relative pronoun what can best be understood if treated as equivalent to that which; whatever can be understood simply as a noun if it does not modify a noun, and the clause beginning with it can be treated as  a noun subject or object, as in Whatever they did was best for whatever happened.  
It not seldom happens that when an adposition is pied-pied, it stays in place where it was; the result is duplicated preposition and postposition. An example from a BBC announcer is: "the situation of which he took advantage of." This phenomenon is especially likely when the clause is complicated. In casual speech, a genitival pronoun is often treated this way; e.g. "the person who I don't like his book." (This is normal in the daughter languages of English.)  It is as old as the Tudor writer Thomas Malory. Where no preposition is involved and no other complications exist, relative clauses prefer that to who or which unless they are embedded in a clause beginning with who or which, or when they are embedded in a relative, nominal clause, or adverbial (purpose) clause beginning with that.

Note that the following conditions have to applied in the order shown. The reason for this is that sometimes more than one condition can apply to the same situation. In such instances, the order of application may affect the outcome, especially when one condition alters the situation that a later condition would otherwise apply to. But consider "Over what they were arguing was about for what purpose it was"; this atrocious example violates condition 2 below as well as the general prohibitions against consecutive prepositions belonging to different clauses and against a a form non-auxiliary of be at the end of a clause beginning with a WH-pronoun (but a clause beginning with whose or a WH-adverb does not resist word-final non-auxiliary be, as already shown).

1. From where? does not occur except in quasi-relative clause beginning with from where.  We do not have direct questions or indirect questions ("They asked me from where I had come") beginning with from where(?). Borderline prepositions like than and as as well participle-derived prepositions (like during, concerning, and according to, as well as the class of than, except, besides, but) are generally foregrounded.  As noticed earlier, a preposition is not postposed when the phrase it belongs to modifies a noun, adjective, or adverb within the clause, unless it would come at the end of the clause if not foregrounded. See more or indirect questions below (at 4).

2. A cleft-sentence (where a word out of the main clause is put first and the rest is subordinated as a that-clause) may overcondition other conditions not permitting a stranded preposition; e.g. It's two stories that (*by which) this building is taller than the other one BY.   While we wouldn’t say the two inches that she's taller than him *BY (see the box above),  the person it was written by is entirely acceptable for reasons given in connection with the next condition—3.

Various examples indicate a greater likelihood--in instances where a preposition may be either foregrounded or stranded--of foregrounding in non-restrictive relative clauses than in restrictive ones.  (This condition is irrelevant to questions, just as 5 is irrelevant to non-interrogative statements.)  
—Metaphorical uses of locative prepositions are harder to strand than uses involving real locations.  We would not say "someone who there is no trust in" but "someone who there is no hope for" is acceptable.

—Note non-restrictive clauses in which adjectival which (or what) accompanies a repetition of the antecedent; e.g. "There is a very likeable old gentleman," "from which old gentleman we received quite a large contribution"; we could also say "old gentleman, which old gentleman we received quite a large contribution from"—though it has got an inelegant aroma.

3. Difficulty (1) in the lower part of the second box from the beginning of this memorandum discusses foregrounding that results in a stranded form of be—just in case be would not be stranded if there were no foregrounding—as in "the one *for whom it was."  Note "the table that it is on" is all right.  The result is vastly worse in either clause of a cleft-sentence; we therefore let condition 2 prevail and say:  What the argument was over is what our discussion is about.    That condition 3 overconditions 4 is not clear from To what extent it was is what they are arguing over—for was is stranded because of a missing so or participle like done.   

4. An indirect question beginning with a WH-word (or in infinitive-construct form) disallows pied-piping; cf. He asked us (*in) what direction we were going in."  Another example is:  "It is not clear what relation they stand in."  But condition 2 overconditions this condition; e.g. "They wondered about in what way that got accomplished." 

5a. It is necessary to strand the second of successive prepositions (excluding participial compounds like according to and participles like except—which would hardly occur in this context) when they belong to different clauses. (This is a situation when a whole relative clause or indirect question having a preposition whose object is a relative pronoun is the object of a preposition in the higher clause.) An example is: "He asked about for what purpose they had planned to do that." That conditions 3 and 4 overcondition condition 5a is clear from the un-English ring of "They wondered about to what extent it was."  This may look like condition 4 is being overconditiond by condition 5a, but the stranding of was here is due to the dropping of a participle like done or a predicate like true or so; if was were not auxiliary or at least copular, one would say existed or some other suitable word. Instead of saying "They spoke about to what extent that was so," it would be preferable to say:  "They spoke about the extent to which that was so." Compare also the pre-emption or overruling of conditions 3, 4, and 6 by condition 3 in "What the argument was over was the question of in what manner the work was carried out"—where out is not a postposition, and where condition 2 does prevail over 3 and 3 is not allowed to set conditions 4 and 5 aside, as should be the case.  The last example is one of the worst examples that result from following the grammar books.

The distinction between prepositional phrases (PP)  that lack their literal senses.  We say "In what way did she succeed?" and avoid "What way did she succeed *in?" as well as "What grounds did they do it on?"—even though we can say "Which list did she put that item in?" (and "In which list did she put that item?") as well as "What table did she put that vase on?" (and "On what table did she put that vase?")   This condition does not apply when a PP stands alone as a question or relative clause; in this instance, the order with postposition following the WH-word is unmarked and normally preferred; e.g. "What for? = Why?").   While we ordinarily avoid P+Nom (e.g. "For *what?"), when necessary substituting fuller phrases (e.g. "For what reason?"; see (d) below, and note that "Who to?" and "What on?" are preferred to "To Whom?" and "On what?" The P+Nom order is used to indicate unusual connotations.   Saying with whom can otherwise sound haughty or rude (being. a put-down as in With whom do you wish to speak?), or just foreign.   However, to highlight a special sense, we can employ the unusual order to marker that sense (see condition 3 below); if invited to sit down but seeing nothing to sit on, one would naturally say "On what?"  This would sound silly otherwise--where we say "What on?"   (See further my "Where English cannot put a preposition before a relative or interrogative pronoun" in The English reference grammar, edited by G. Leitner [Niemeyer, 1986, pp. 156-77].).  For what (cause/reason) markers a special situation that would not be markered with What for?

5b. The converse of condition 6 is true:  When successive prepositions belong to different clauses, the first one cannot be postposed, as in an extent (that/which) we were not aware of *to—for proper an extent to which we were not aware of.

6. Consider partitive and similar prepositional phrases consisting of a preposition + quantifier + of (occasionally, some other preposition) + whom or which¾ like those found in "the candidates few/some of whom had spoken here." More stilted but still acceptable, and in complex dependent clauses sometimes preferred, is "The candidates of whom few/some had been given permission to speak here," but not "the candidates few/some of whom permission to speak here had been given to."   Foregrounding is, however, blocked when  condition 5c prevents a succession of non-conclausal prepositions.

We cannot postpose the preposition of prepositional phrases whose object is a relative pronoun when it modifies some item within the clause rather than the whole clause--i.e. the main verb or an infinitive depending on the main verb--unless that word comes at the end of the clause.  Though it is all right to say one who I went there with, we prepose with in one with whom a dialog would lead nowhere."   But we do say the one who she enjoyed speaking with, where speaking is a nominal--a gerund.  Similarly, an adverbial phrase that modifies an adjective or participle or adverb within the clause rather than its verb predicate is not postposed unless that item ends the clause.    So we have got to say for whom it was urgent to protest when for whom modifies urgent; but who it was urgent to protest for is all right if for whom modifies the verb form (infinitive)—to protest.   If the prepositional phrase whose object is a relative modifies a noun, adjective, or adverb within the clause and also happens to come at the end of the clause, there is less of a problem with postposing its preposition; cf.  one who they recognized his similarity to and one they recognized her being similar to,  one who they recognized Jim as having been attaked by, and one who they saw us walking leisurely with (where with whom modifies walking, which serves as the predicate of us walking leisurely).  This is less recommended with a comparative preposition; cf. one who they recogmized their being as happy as and one who he did that more quickly than. 

     When what introduces an indirect question or modifies a noun, it is not equivalent to that which.  (The same is also of course true of determiner what in "They knew what newspaper th’others had read that in.") To avoid confusion, observe that "Many of high standing, but not everyone who was, attended" is a awkward, though it violates no conditions; it is made better if the who-clause is set off with dashes: "Many of high standing--but not everyone who was--attended.")  The grammar can become very complicated when a relative clause is embedded within another; cf. "the program <that you were determined to involve all of the projects {that they came up with} in>."  (The curly brackets bound most subordinate clause, while the pointy brackets mark off the relative clause it is embedded in.) This is a situation in which stranding should be avoided if possible. On the other hand, pied-piping can be a put down, as in "…." If a WH-adverb is replaced with a phrase like "For what purpose did you do that?" the effect can be condemnatory in some contexts. In simple sentence fragments contain only a preposition and WH-word, the order is quite important. "For what?" is haughtier or at least archer than normal "What for?"

    Foregrounding a preposition to the front of the clause is sufficiently un-English in most instances that we hear announcers on the BBC and American media saying things like the situation of which he took advantage of.  Even the Tudor writer Thomas Malory committed this kind of error.   Where no preposition is involved--and no complications or weird senses would result—we can omit who and use that instead—as in the one [that] I saw.   Note that relative that …xx.

Note that in good English we avoid a vocative noun followed by who and a second-person verb.  It is difficult to say, "Lord, Who take away the sins of the world."  Colloquially, the verb is generally a third-person verb when non-vocative you or I or me is not the antecedent of the relative, as in  "You are the Lord, Who takes away,"  "It's me who does things on time."   It is as though one were thinking, "Lord, You Who are the one that takes" and "It's me who is the one that, etc."

SEE ALSO HERE