That’s a chain stitch, formed by a cover stitch machine. Yah, I’m assuming there is a specialized flat fell machine that does all the work in one pass. The shirt is probably fed ‘flat’ and the feller does the two parts at once. A double needle with loppers to form a chain stitch on the bottom. I just noticed on my DH’s brooks brothers it’s the same as yours. His older Ralf Lauren has one seam on the outside and two on the inside - which is formed with two passes. His newer ones has the double chain stitch.

I think I get your question now. On the store bought shirt, can you tell me if the left side seam has two rows of top stitching or one? I only see one - which if this is the case I have an easy answer (one side is sewn RST for the first pass, the other side is sewn wrong sides together for the first pass). But if it is not the case I’m going to investigate more.

Hi Brian! Thanks for reaching out. When sewing the side seams (even if your aren’t intending to flat fell them), we always sew from hem to cuff - think of it as sewing down the rabbit hole. When doing a flat fell side seam, you sew the seam in two passes, both passes originate from the hem. The basic steps are below. I’ve attached a link to a short video tutorial on the method. Hope this helps! 1) attach flat fell foot 2) with right sides together, matching underarm seams, offset the underside of the shirt side seam to be 1/4” wider than the top side. The underside will feed into the feller. 3) stitch the first pass from hem to cuff. 4) open shirt as far as possible, so it is flat against the machine. 5) sew second pass again from hem to cuff, feeding previously felled seam allowance into feller so it folds again over the shirt. Walk over bulky seams as needed - you might need to remove the goods from the felling foot at the underarm seam. https://vimeo.com/awaywesew/sideseams-flatfell

It definitely easier to stitch the squares off style, but for top stitch the round, I recommend drawing the top stitch line with a frixtion marker and following that. I also use an edge stitch foot, but that takes a little practice. Grab a piece of card stock or an index card, cut it to match the round shape, and practice stitching. Check stitch length (adjust smaller around curves), and otherwise audition the top stitching. It can also help to use a smaller needle. Sounds opposite, but it gives more precision.