The Joint Resolution for Annexing Texas to the United States,[1] approved by Congress on March 1, 1845, states:

Third – New States of convenient size not exceeding four in number, in addition to said State of Texas and having sufficient population, may, hereafter by the consent of said State, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the Federal Constitution; and such states as may be formed out of the territory lying south of thirty-six degrees thirty minutes north latitude, commonly known as the Missouri Compromise Line, shall be admitted into the Union, with or without slavery, as the people of each State, asking admission shall desire; and in such State or States as shall be formed out of said territory, north of said Missouri Compromise Line, slavery, or involuntary servitude (except for crime) shall be prohibited.