Illinois Route 37 connects Effingham with Cairo, running through places like Marion in the backbone of southernmost Illinois. It parallels Interstate 57 for its entire length; south of Effingham, Interstate 57 switches from following US Route 45 to following Illinois 37 through the rest of Southern Illinois. It runs north-south overall, with a bit of a southwesterly bent.
Illinois 37's northern end is near Watson, just south of Effingham, at US Route 45:
Illinois 37 meets the eastern end at Illinois 185 in Farina: