Article III, Section 3:
The Trial of all Crimes, except in Cases of Impeachment, shall be by Jury; and such Trial shall be held in the State where the said Crimes shall have been committed; but when not committed within any State, the Trial shall be at such Place or Places as the Congress may by Law have directed.
The 6th Amendment provides:
In all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial, by an impartial jury of the State and district wherein the crime shall have been committed, which district shall have been previously ascertained by law, and to be informed of the nature and cause of the accusation; to be confronted with the witnesses against him; to have compulsory process for obtaining witnesses in his favor, and to have the Assistance of Counsel for his defence.
In 1789, before the 6th amendment was ratified, it would seem a criminal defendant would have had the right to trial by jury. That trial will have been held in the state wherein the crime was committed.
The 6th amendment also guarantees the right to trial by jury, and requires that the trial be held in the state (and distract–thats new) where the crime was committed. The 6th amendment adds the right to a speedy and public trial, requires that the jury be impartial (though I’m not sure what good a partial jury is), and some other stuff.
So is part of the 6th amendment redundant if Article III already protected these rights? I don’t know. I just had never noticed this duplicative language before.
Update: Thanks to tipster Mike below. John Marshall Harlan (who else) addressed just this question in Callan v. Wilson in 1888. Harlan FTW.