Not all parts of the "Southwest" counties that border the "South" counties would I consider to be in the Southwest. If most of the county fell west of a straight line drawn down from the border of Tarrant and Parker county, then I highlighted that county.
This map showing the line dividing the Southwest from the South is more precise. According to this, if driving I-35 you would cross into the Southwest as you enter Williamson county. From I-10, you would cross over as you enter Caldwell county. From HWY 190, you would cross over about 5 miles west of the I-35/190 intersection.
The expression that Dallas is where the East ends, and Fort Worth where the West begins seems to be pretty much true haha...not just for cultural reasons but for geographical reasons as well.
