Sewing Rule: Always backstitch the top and bottom of an on-seam pocket opening, regardless of which type of pocket it is- separate or extension style.
When sewing an on seam pocket, the standard practice is to stitch the top portion of the seam (housing the pocket) first, then move to the bottom portion of the seam, stitching it separately. The end of the first stitch (top) will correspond to the top corner of the pocket opening while the beginning of the bottom seam stitch will correspond to the pocket opening bottom corner. It is imperative to backstitch for. longer portion and even a couple extra times to ensure these two locations are properly reinforced for long term durability.
If you take a look at the wrong side of the pocket application above, you should see the top and bottom seam stitches that create the pocket opening. These stitches have backstitching at each end to ensure enough durability at the pocket opening.
In a separate on seam pocket application, the seam stitch including the backstitching should be placed slightly to the left of the stitch connecting the pocket pieces to each corresponding seam edge. Doing so will ensure that the pocket bag layers do not accidentally get caught within the garment seam, showing on the face of the application. This placement ensures that the top and bottom of the seam transition seamlessly into the on-seam pocket opening.
You should also backstitch at each end of the pocket opening when sewing an extended on-seam pocket technique. In the image above, you'll see that the top seam stitch and the bottom seam stitch both have backstitching within the pocket bag area, creating the pocket opening.
Reinforcing the pocket opening corners with backstitching is imperative in the construction of an on-seam pocket taht withstands continuous use throughout the garment's lifespan. The backstitching doe snot only prevent the seam from coming undone and becoming loose, it also allows enough strength to withstand's the hand's width and motion.