TN 12 Originally Clarksville to Nashville 1923 Official

TN 12 extended to Kentucky 1926 Official

In late 1926, US 41 was assigned to the entirety of TN 12.
By fall 1927, TDOT was signing US 41 as US 41W instead.
Between 1934-39, TN 12 was removed from John Mallette Dr in Bordeaux.
By 1939, TN 12 was removed from Old Hydes Ferry Pike in Scottsboro and abandoned routing east over to today's TN 155.
Also by 1939, TN 12 was removed from Golf Club Ln in east Clarksville and placed on TN 76 (now TN 112) Madison St.
Also by 1939, TN 12 was removed from Old Mill Rd (no longer connects to)/Tobacco Rd northwest of Clarksville
It appears TN 12 was removed from Old Hopkinsville Hwy (cutoff by TN 374) in the early 1940s.
Also in the early 1940s, TN 12 was rerouted in Nashville. Originally it ran from 5th and Deaderick St in downtown Nashville, up 5th to Monroe St west to 9th north (through Kroger parkng lot now and also does not cross I-65) to Buchanan west to 18th (now Dr. Todd Blvd) to Clarksville Pile across the river with current US 41A. The new routing began at Broadway and 8th, running up 8th to Clay west to 18th north to Clarksville Pike.
It appears TN 12 was removed from Old Hydes Ferry Pike and Caldwell Rd east of Ashland City in the mid-1940s
In 1949, TN 12 was removed from Chapmansboro Rd west of Ashland City

TN 12 multiple RR crossings 1932 Official

TN 12 north of the RR everywhere between Clarksville and Nashville 1951 Official

By 1954, TN 12 was placed on one-way splits in central Clarksville - SB used the original route of 2nd St while NB used 3rd St/College
In 1955, TN 12 was removed from W Chetham School Rd near Cheap Hill; Bethel Rd/Mosley Ferry Rd/Old Hwy 12 in the Bethel area
In 1958, US 41A was placed on Robertson Pkwy in downtown Nashville, removing it from 8th Ave between Church St and where it leaves Robertson Pkwy. Because no other route was sent this way at first, TN 12 may have been extended south on Robertson Pkwy to 8th/Church until US 31W-431 (and thus, TN 6) was moved to Robertson a bit later.
Around 1959-60, TN 12 was removed from Edwards Rd; Old Ashland City Hwy through Fredonia; E. Leonard Rd; Earl Rd
Between 1957-64, TN 12 south was truncated to the new James Robertson Pkwy
In the mid 1970s(?), TN 12 was removed from E Old Ashland City Rd north of Excell; also placed onto Riverside Dr around the south and west of Clarksville, leaving behind more Old Ashland City Rd, the remaining bit of Golf Club Ln it was using; Madison St, 2nd/3rd one-way pair between Madison and College; N 2nd St.
Between 1973-75, TN 12 was rerouted in Nashville to use Metrocenter Blvd (now Rosa Parks Blvd) off of Clarksville Pike, leaving behind some Clarksville Pike and Clay St.

TN 12 original Nashville routing 1930 Official

TN 12 rerouted to 8th Ave 1951 Official

TN 12 rerouted to use Rosa Parks Blvd 1975 Davidson County

Around 1999, TN 12 was removed from Old Ashland City Hwy west of Scottsboro.
Posting: TN 12 is primary throughout except Robertson Pkwy and I-65 in downtown Nashville, which is secondary.
TN 12 is not posted south of the US 41A split in Bordeaux nor is it posted on its US 41A Bypass and 41A overlays in Clarksville.
Secret US Route Overlay: US 41 ALT I (4.3 mi)
US 41 ALT Byp (4.5 mi)
US 41 ALT II (6.9 mi)
Comment: TN 12 provides a backdoor way from Clarksville to Nashville