The Mattress Stitch is a strong, nearly invisible method of joining two pieces of knitted fabric. I use this method most often; and it is the best method for seaming side and sleeve seams of garments. Seaming is made easier if you have blocked the pieces.
The mattress stitch is worked from the right side and joins the pieces row for row. This is why counting rows is important. The seam is invisible on the right side and looks like a continuous piece of knitting.
Mattress Stitch (Invisible Weaving) on Stockinette Stitch
If you examine the edge of stockinette stitch you will see a ladder of “running bars” between the “V”’s or knit stitches. The sides are sewn together by stitching around these bars. With the wrong sides facing, begin by joining the edge stitch. Insert threaded needle under the first two horizontal bars on one piece, then insert the needle under the corresponding two bars on the opposite piece. Continue alternating from side to side, pulling the yarn firmly every few stitches. You will notice the pieces lining up for an invisible join on the right side. You can also work one bar on either side; it just takes longer to sew. The pieces may be off by a few rows, so compensate for this by picking up one bar on one piece and two bars on the opposite side to even out the tension.
Mattress Stitch (Invisible Weaving) on Reverse Stockinette Stitch and Garter Stitch
With these two pattern stitches the right sides have “bumps” or purl stitches instead of the “V” stitches. The mattress stitch is worked through the bottom loop of the “bump” on one piece and then through the top loop of the “bump” on the other piece. Start the same way as for stockinette stitch, that is with wrong sides facing and join the bottom edge. Next insert the needle into the top loop on one side then into the bottom loop of the corresponding stitch on the other side. Continue up the seam alternating in this way. It doesn’t matter if you begin under the bottom or top loop, just make sure you alternate loops as you work the seam. Keep as close to the edge as possible with garter stitch. You will find this seam worked on garter stitch fabric is less bulky on the inside than with stockinette stitch fabric. The edges just meet.
Ribbing can be joined with the mattress stitch; choose either invisible weaving for stockinette stitch or garter stitch depending on whether the edge stitch is a knit or purl stitch. I prefer to join ribbed edges by overcasting on the wrong side and then work the mattress stitch from the right side for the rest of the seam.