By force of the Fourteenth Amendment, a citizen of the United States residing in a state of the Union becomes a citizen of that state. A citizen of the several States domicile in an individual state becomes by Article IV, Section 2, Clause 1, a citizen of that state.

Therefore, a citizen of the United States and a citizen of the several States, being citizens of a State, can under the Constitution of the United States, at Article III, Section 2, Clause 7 (Citizens of different States), sue one another if they are a citizen of a different State of the Union.