Read & Mullin is a commercial site, so we can't put in a link, but they have a short explanation. For more than you ever wanted to know about the Mason-Dixon Line (Maryland is genereally dropped for Southern collectors):
https://en.m.wikipedia.org/wiki/Mason–Dixon_line
You might say it was all ex-Spanish cowtowns beyond New Orleans, but Texas is usually accepted as the cut for a western border as there's no one left from the nineteenth century territories to argue the point.