During the Constitutional Convention, the concept of slavery was a very sensitive subject.  Many members at the convention had slaves of their own, as did the people they were representing.  The Constitution does not prohibit slavery.  This was necessary for the ratification of the Constitution, as the large southern states would refuse to ratify the Constitution were it to have such a restriction.

Even within the Constitution the word slave or slavery is not mentioned.  The Constitution often refers to slaves as other persons, as it does in Article I, Section 2 of the Constitution.  In this passage, it states that representation in the House of Representatives would be found by adding to the whole Number of free Persons three fifths of all other Persons.  This is often referred to as the three-fifths compromise, which allowed slaves to count as three-fifths of a person.  The writers made this accommodation to appease the large southern slave states, whose slaves totaled over one-third of the population.  Later, in Article IV, Section 2, another reference is made that basically states what would later be known as the Fugitive Slave Law, in which runaway slaves must be returned to their owner if found.  Both indirect but obvious references to slavery were made to sway the slave states to ratify while not deterring the smaller non-slave states.