Giles County Alabama State Route 127 becomes
SR 166 at the
Alabama state line in the
Bethel community of southern
Giles County. SR 166 goes north as a 2-lane highway and intersects
SR 273, and then goes north through rural and hilly terrain before coming to an intersection with
US 64 and entering
Pulaski. SR 166 then becomes concurrent with
SR 11, widens to an undivided 4-lane highway, and crosses a bridge over
Richland Creek to enter downtown and come to an intersection with
SR 15, where SR 166 splits from SR 11 to follow SR 15 west as a 2-lane. They then leave downtown and comes to another intersection with US 64, where they become concurrent with US 64 and SR 15 becomes unsigned. They then leave Pulaski and continue west as a 4-lane undivided highway for a little over . SR 166 splits off from US 64/SR 15 and turns northward as a 2-lane highway. SR 166 then passes through rural areas before passing through
Campbellsville and having an intersection with
SR 245. It then continues north to cross into
Maury County.
Maury County SR 166 continues northwest to enter
Mount Pleasant, where it intersects and becomes concurrent with
SR 243. They then enter downtown, where SR 166 splits off and goes west to have an intersection with
US 43 (
SR 6). SR 166 then turns northwest to leave Mount Pleasant and pass through rural areas before entering
Hampshire and coming to an end at an intersection with
US 412 (
SR 99). ==Major intersections==