It's a bar tack and secures a seam at a point of stress. http://en.wikipedia.org/wiki/Bar_tack Check your shirt from the inside. You see the folded over section is hand-stitched from the hem up. Higher up that section is no longer stitched-down but held just by the buttons. If the bar tack wouldn't secure the point where the stitching ends, you might accidentally rip the seam apart.
8/26/14 at 6:49am