Tennessee Code Title 8. Public Officers and Employees § 8-24-101

(1) Counties having a population of four hundred thousand (400,000) or more shall constitute counties of the first class;

(2) Counties having a population of one hundred fifty thousand (150,000) or more, but less than four hundred thousand (400,000), shall constitute counties of the second class;

(3) Counties having a population of fifty thousand (50,000) or more, but less than one hundred fifty thousand (150,000), shall constitute counties of the third class. Within the third class, counties having a population of seventy-four thousand five hundred (74,500) or more shall constitute subclass A, and counties having a population of less than seventy-four thousand five hundred (74,500) shall constitute subclass B;

(4) Counties having a population of twenty-three thousand three hundred (23,300) or more, but less than fifty thousand (50,000), shall constitute counties of the fourth class;

(5) Counties having a population of twelve thousand (12,000) or more, but less than twenty-three thousand three hundred (23,300), shall constitute counties of the fifth class;

(6) Counties having a population of five thousand five hundred (5,500) or more, but less than twelve thousand (12,000), shall constitute counties of the sixth class;

(7) Counties having a population of three thousand seven hundred seventy (3,770) or more, but less than five thousand five hundred (5,500), shall constitute counties of the seventh class; and

(8) Counties having a population of less than three thousand seven hundred seventy (3,770) shall constitute counties of the eighth class.

(b)(1) The population of counties for purposes of this section shall be determined by the 1970 federal census and the most recent succeeding federal census.

(2) Except that the population of counties which have a population of not less than thirty-four thousand four hundred (34,400) nor more than thirty-four thousand five hundred (34,500), according to the 1970 federal census or any subsequent federal census, for purposes of this section shall be determined by the 1970 federal census and any succeeding census taken in the county or counties.