I think it would be nice to refer to the GADM system.
Database of Global Administrative Area (GADM) is spatial database of the location of the world’s administrative areas, which including 294,430 administrative areas in about 250 countries (virtually whole world). Most administrative areas in GADM classified into three hierarchical levels.
In the United States, there are classified into three levels. 1 - Nations(eg US), Level 2 - States(eg Texas), and Level 3 - Counties(eg Houston)