Article IX is quite narrowly focused on the formation of counties, an important matter for a frontier state like Texas in the nineteenth century. After granting the Legislature the power to create counties "for the convenience of the people," this article lists a series of restrictions. New counties created outside the counties that already existed had to be no smaller than 900 square miles and "shaped in a square form." The framers understood that this may not be possible in border areas, and allowed that, in such cases, the area could be less.
The framers also recognized that geographically large counties may seek to split into smaller counties. New counties created from existing counties could be no smaller than 700 square miles. However, they also must not be "nearer than twelve miles of the county seat" of any county from which the new county's territory is taken.
This article provides additional details on the location and removal of county seats - the main concern being that the county seat should be roughly in the geographic center of the county (within five miles of it).