Countries such as India and Indonesia produce a lot of garments that are French seamed or Flat felled seamed.  From what I understand it is mainly because the company producing the garments either contracts out the work to seamstresses/machinists who do not have over lockers, or they don’t have over lockers in their factories.  I have seen documentaries where villages in remote areas use the old Treadle machines because they do not have electricity.

Technology has come a long way in the Western World, but it hasn't caught up to some countries, and many garments are still being sewn on these very old sewing machines.

What this means when it comes to altering the garment, is that the seams are not the standard stitch and over lock.  Therefore the seams have to be unpicked, pinned back to the original position so you have the reference point, and convert it to a normal stitch and over lock.  I have seen people attempt to take in jeans with a flat felled seam at the top section only.  Trying to refold can be difficult.

If you are taking in the back of a pair of jeans or casual trousers, I would recommend unpicking the complete center back seam to the crotch, re pin the seams so that they are in the same position as before, then mark how much you are taking in, sew the new seam, cut off the excess and over lock the edge.  I cut off the excess and then over lock, rather than have the over locker cut the fabric, because some denim is very thick, and some times the cutting blades can’t take the thickness.  You can then re sew the double rows of stitching on your domestic sewing machine, by attaching two threads coming from the top and down into the needle, then sewing what I call topside. 

Happy sewing

Judith aka genie