Topstitching the inside AND outside seam of pants

Question

I have cotton cargo pants (from Old Navy, I think) where the outside seam has two topstitches and the inside seam only has one. Even the seam between the left/right halves has a topstitch.

How is that possible? What tool or technique was used to get a topstitch on every major seam?

The only thing I can imagine is a sewing machine where the needle sews parallel to a long, thin base, allowing each pant leg to be topstitched after being joined with a stitch when they were inside out.

Here are pictures of the stitches for a single leg of my pants. Note how one is double topstitched, and the other is single. The point is that all stitches on the pants are like this (with no breaks), so I don't see how they did it unless they have a special machine.

^{Click images to enlarge}

Answer 1

Since this was mass-produced clothing, they probably do have a special machine for it! You can see an example in action around 2:45 in this video "How It's Made: Jeans". They're using a machine like this one, the Singer 261u:

You can see how the arm is shaped in a way that would allow you to scrunch up the leg.

You could try topstitching like this on a typical home sewing machine, but success would depend on the thickness of the fabric and how skinny the legs are... either way, take it slow!!

Alternatively, a common finish on pants is to only topstitch one set of seams. You can easily do this on a typical home sewing machine! For example if you want topstitching on the outside, sew and topstitch the outer seams before doing the inside seams. This way you have plenty of room to maneuver. Then you can complete the inside seams.

Another option is to topstitch the inside seams, and then partially topstitch the outside seams, usually just past the pocket - again, easy enough to do on a typical machine. You can see a tutorial for this sort of assembly here.

Answer 2

It's called a welt seam. You can sew seams w/o finishing the edges with an overlocker because they're kept within the seam