Tennessee Formation Maps web site for those researching the state. At the top of the web page is a section called Interactive Maps from 1776 to 1985. You can play the maps, go back and forth between years and view specific years. I recommend that you play the maps and then view each separately.
To enhance your knowledge of Tennessee county formations, read the chronology in text. This begins with Washington District of North Carolina which was formed 12 November 1776. There is also a list of Tennessee discontinued counties. I had never heard of Spencer County which was formed in 1785 from Sullivan County and renamed Hawkins County in 1786. But I did know about Sullivan and Hawkins counties. No genealogist likes to even think about destroyed records, burned or ravished courthouses, but it happens. There is also a list on the web page of Tennessee destroyed courthouses.
A study of the chronology also reveals the changes in government in Tennessee. In 1784 three counties, Washington, Greene and Sullivan, formed the provisional State of Franklin. If your ancestors lived in northeast Tennessee in that time period, the records may refer to them as being in the State of Franklin. Three years later North Carolina gained supremacy over the State of Franklin and once again they were part of North Carolina until ceded to the federal government. A document that I have refers to my ancestor as living in the "Territory of the United States South of the Ohio River." That could encompass a broad area, but when named in 1790 it was in reference to what would become the state of Tennessee. Finally on 1 June 1796 Tennessee was admitted as a state.
By reviewing the maps and county formations, you can place your Tennessee ancestor in the proper time period and place. This one web page is outstanding for genealogical information!