Which was an important part of the northwest ordinance.

The Northwest Ordinance was important for two major reasons.  The first of these was that it banned slavery in the territories of the Northwest.  This ensured that these would be free states when they entered the Union.  But that is not the most important thing about this law.  The most important thing is that it created a system whereby territories could become states

Explanation:

 This meant that there would be no situation in which (for example) the Ohio region could be a colony of Virginia.  This meant that the US would not have to worry about fights between states over land or about colonists in new territories feeling abused by whichever state was their "mother country."  This allowed the US to expand in an orderly and stable way.

It allowed the ownership of slaves in all new territories.

Explanation:

uncle sam would be tossing and turning in his grave : *)