[jmacusa]

For the life of me I can't find where in the US Constitution slavery is allowed.

[BroJoeK]

jmacusa: "For the life of me I can't find where in the US Constitution slavery is allowed."

Seriously?
A quick search produces two oblique references:

1. Article 1, section 2: "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."

   "Other Persons" means slaves, the 3/5 rule.

2. Article 4, Section 2: "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."

   This is the fugitive slave clause.

There may be other references, but I can't think of them now...

The key point to "get" here is that without such assurances, Southern delegates to the Constitution Convention and to state ratifying conventions would not have agreed to join the United States.
Had they walked out, we would likely have seen several loosely affiliated countries in North America, and frequent warfare amongst them.