Article I, Section. 2 [Slaves count as 3/5 persons]
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons [i.e., slaves].
Article I, Section. 9, clause 1. [No power to ban slavery until 1808]
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight
, but a Tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.
Article IV, Section. 2. [Free states cannot protect slaves]
No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
Article V [No Constitutional Amendment to Ban Slavery Until 1808]
...No Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article.