Duncan v. Louisiana was a case ruled by the Supreme Court after Duncan appealed, claiming that his Sixth and Fourteenth Amendment rights were violated, and that he should have had a jury trial.
The ruling was that jury by trial is within the Fourteenth Amendment, and for this reason, is applicable to the states.
In conclusion, the Supreme Court ruled that the Sixth Amendment could be incorporated to the states, and the clause that allowed this incorporation to take place was Due Process.