Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation.
It's a line from Martin Luther King's famous speech, I wonder if it is inverted?
Because I suppose normally we would say, we stand in his symbolic shadow today.
Why is his sentence structured like that?
Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation.
"In whose symbolic shadow we stand today" is a subordinate clause and specifically a relative clause.
We could instead say "whose symbolic shadow we stand in today" - this preserves the normal word order of the phrase "stand in", as in "standing in (someone's) shadow".
But in formal usage you quite often see the preposition moved forwards so that it precedes the relative pronoun ("whose", "whom", "which").
So "the glass that I drank from" can also be phrased as "the glass from which I drank", and "the person that I gave the book to" can be worded as "the person to whom I gave the book". This sounds stilted (overly formal) in everyday speech. In everyday speech it's much more normal to have the preposition at the end.
If you are asking why a relative clause is used at all:
You could say "we stand in his symbolic shadow today" (without "whose") but this would then be a separate sentence, although it could be parenthetical (within brackets or dashes): "Five score years ago, a great American (we stand in his symbolic shadow today) signed the Emancipation Proclamation."
The use of a relative clause makes the sentence flow better than a parenthetical remark would, though, and the elevated or literary tone of "five score years" fits with the literary style of the rest of the sentence (including "in whose").
He’s speaking of Abraham Lincoln, who signed the Emancipation Proclamation. But, more subtly, he’s also talking like Lincoln did. King used the archaic phrase “five score years ago,” instead of “one hundred years ago,” as a reference to a famous speech by Lincoln, the Gettysburg Address. It begins with the words, “Four score and seven years ago.” King was expecting his audience to recognize this, but it might be obscure to someone from a different culture.
There’s already a good answer by @rjpond that explains the grammar: “in whose symbolic shadow we stand today” is a relative clause. King is saying that we stand in the symbolic shadow of the great American who signed the Emancipation Proclamation.
I will add that, in formal written English back in the nineteenth and twentieth centuries, it was often considered a grammatical error to end a sentence or clause with a preposition, even though that was how people normally spoke. In Lincoln’s and King’s times, “in whose symbolic shadow we stand” would have sounded more formal and “whose symbolic shadow we stand in,” more casual. This phrasing also recalls the more old-fashioned way Lincoln spoke, for example, “... He gives to both North and South this terrible war as the woe due to those by whom the offense came ....” (From his Second Inaugural Address, emphasis added.)
Linguists no longer think there’s anything wrong with putting a preposition at the end of a sentence, instead of before the pronoun, but people still sometimes say things like “to whom” for poetic effect.
It's structured that way because it is a single sentence, not several; it echos Abraham Lincoln; it comes close to being poetic.
We normally would not - could not, even - say in that way "… we stand in his symbolic shadow today". That would require two sentences, at least.
Changing the one sentence into two starts to reduce the sentiment from near-poetry to mere fact. Replacing "in whose" with "his" makes that worse.
