The Lexington barbecue sandwich is defined by a single cut and a sauce thin enough to drink. This is the North Carolina Piedmont style: pork shoulder and only shoulder, smoked over wood and chopped or coarsely sliced, then dressed with a sauce known locally as dip. The dip is the eastern vinegar-and-pepper base with one change, a small measure of tomato or ketchup worked in, which tightens it into a thin red liquid rather than the near-clear eastern dressing. That tomato is the entire dividing line. East of here the sauce has none and the whole hog goes in; in Lexington it is shoulder and a red dip, and the sandwich is built so a thin sauce can do its work.
The craft is in what shoulder gives that whole hog does not, and in what a thin sauce does that a thick one cannot. Shoulder carries enough fat and collagen to survive a long smoke and finish as the texture of the meat rather than rendering to string, and using one cut means a consistent dark, even chop instead of the blended balance of the whole animal. The dip is poured and worked through the warm meat as it comes off the board, and because it is thin it penetrates the chop and seasons from the inside, the acid cutting the shoulder's richness rather than glazing it. The bun is plain, soft, and meant to vanish, soaking the loose dip without going to paste. The cap is red slaw, cabbage dressed with the same vinegar-tomato dip instead of mayonnaise, so the slaw is structural and on-theme at once: cold, sharp crunch tinted the same color as the sauce, doing the job a creamy white slaw does elsewhere without breaking the flavor line.
The variations sit along the same regional border and are codified rather than loose. The eastern North Carolina build steps to whole hog and drops the tomato entirely; the South Carolina lineage swaps the dip for a mustard sauce; the generalized pulled pork sandwich is this shoulder build with the regional dressing left open. Each of those is its own argument with its own defenders, and each deserves its own article rather than being crowded in here.