Article I, Section 2, Clause 3 (the "three-fifths clause") gave slave states representation in Congress based on 60 percent of their slaves:

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. (Note: The part of this Clause relating to the mode of apportionment of representatives among the several States has been affected by Section 2 of amendment XIV, and as to taxes on incomes without apportionment by amendment XVI.)
 

Article I, Section 2 (above) and Article I, Section 9, Clause 4 (the "capitation tax clause") limited the potential taxation of slaves:

No Capitation, or other direct, Tax shall be laid, unless in Proportion to the Census or Enumeration herein before directed to be taken. (Note: This Clause has been affected by amendment XVI)
 

Article I, Section 9, Clause 1 (the "migration and importation clause") prohibited Congress from ending the African slave trade before 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 V (amendment provisions) gave added protection to the slave trade by prohibiting any amendment of the migration and importation clause before 1808 and by requiring 3/4 of the states to assent to any constitutional amendment (guaranteeing that the South could always block any proposed amendment):

The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that 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; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.
 

Article IV, Section. 2, Clause 3 ("fugitives from labor clause") provided for the return of fugitive 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. (Note: This Clause has been affected by amendment XIII.)

Other clauses strengthened slavery by:

--guaranteeing that federal troops would be used to suppress slave rebellions

--prohibiting tax on exports (which would have allowed for the indirect taxation of slaves)

--giving the slave states extra votes in the electoral college under the three-fifths clause