The border is obviously hazy, as anyone familiar with the events of 1861-65 can attest. The five most widely used borders are the Rappahannock River, the Potomac River, the Ohio River, the Mason-Dixon Line, and U.S. Route 40. Each of these can seem equally logical and preposterous depending on what kind of metric you're using. Here are some of the best ways decide:
Surveys and Censuses
The Mason-Dixon Line is the most traditional border between North and South, and to some extent the line made sense in its time. Maryland was a slave state, home to the likes of Frederick Douglas and Harriet Tubman, and Lincoln had to send federal troops into Baltimore to quell secessionist riots -- all suggesting Maryland was a southern state.
The Line endures today and the U.S. Census still lists Maryland and D.C. as part of the South. In fact, the Census even calls Delaware southern, which seems a bit misguided. The concept of the Mason-Dixon Line today is outdated, as few people would describe Baltimore, with its ethnic neighborhoods and industrial tradition, as southern.
Many historians and sociologists decided long ago that the Mason-Dixon Line was too clumsy and that U.S. Route 40 -- the old National Road -- was a more accurate border. The road extends from Baltimore to Frederick to Cumberland, through Wheeling, across southern Ohio, through Columbus and Indianapolis, across southern Illinois, and out to St. Louis.
In the "Nine Nations of North America," Joel Garreau noted that there are "substantial differences in food, architecture, the layout of towns, and music to either side of that highway." Southern Indiana, he wrote, "is definitely part of Dixie, and has been ever since the Coppherheads (those Northerners who sympathized with the Confederates in the 1860s)."
Gen. George McClellan could never cross the swampy Chickahominy River outside Richmond, and so everything south of there is clearly property of Dixie. But a more frequently-used border is the Rappahannock, which is about halfway between Washington and Richmond. Most neighborhoods north of the Rap feel metropolitan while counties south are rural.
The Potomac was also the effective border between the USA and CSA. The Feds' decision to coin the Army of the Potomac was symbolic, as it hinted at the central point. Similarly, the Army of the Ohio suggested that the Ohio River was the western border between North and South, which seems reasonable if you consider Kentucky southern and Ohio northern.
If you look at the Kentucky/Ohio and Kentucky/Indiana borders, you'll also see that the southern state is overwhelmingly Baptist while the northern one is a mix of Catholics, Methodists, and Presbyterians. Not surprisingly, the Baptist counties in southern Illinois supported Stephen A. Douglas (who founded a Baptist seminary) over Lincoln, who was a Presbyterian.