The Doctrine of the Equality of States, (also called Equal Footing), is based on Article IV, Section 3, Clause 1 of the U.S. Constitution, which says:
“New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.”
Additionally, since the admission of Tennessee in 1796, Congress has included in each State’s act of admission a clause providing that the State enters the Union “on an equal footing with the original States in all respects whatever.”
North Carolina ratified the Constitution on November 21, 1789 and became the 12th state to be admitted to the United States. Rhode Island became the 13th state when it ratified the Constitution on May 29, 1790. Both North Carolina and Rhode Island were admitted on equal footing with the previously admitted states. Even though North Carolina and Rhode Island were not states when the proposed twelve amendments were submitted to the states on September 25, 1789 they were allowed to vote on the ratification of the Bill of Rights.
When Vermont became the 14th state March 5, 1791 they were also admitted on equal footing and voted for the ratification of the Bill of Rights. Two decades later in 1810, Congress approved of the 13th Amendment and it was sent out to the states for ratification. At that time there were 17 states and the approval of 13 would be necessary to ratify the amendment.
On April 30, 1812 Louisiana became the 18th state to be admitted to the Union and they were entitled to vote for or against the ratification of the proposed 13th Amendment. The number of states necessary to ratify the amendment increased from 13 of 17 to 14 of 18.
Since only 13 states ratified the proposed 13th Amendment, it failed to receive the approval of enough stares to be lawfully ratified.