The mattress stitch is a strong, nearly invisible method of joining two pieces of knit fabric. I use this method most often; and it is the best 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. If you have left a long tail from the cast-on row, use this to begin sewing. For a neat join with no hole, insert the yarn needle back to front through the corner stitch of the piece without the tail; make a figure 8 and insert the needle in the same way into corner stitch with the yarn tail. Tighten and continue with the following instructions. 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 “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.
Other Seams Worked From Right Sides of Fabric
Invisible Horizontal Seam
This seam is used to join two bound off edges, such as shoulder seams. There must be the same number of stitches on each piece (ie. the same number of stitches at the shoulders for both front and back pieces). This seam is worked stitch by stitch, and can be used with most pattern stitches, not just stockinette stitch, as presented in the image below. The bound off edges do not have to be straight, so this seam works just as well with sloped shoulder edges, as long as the stitch count is the same for each edge.
Pin shoulders or other bound off edges with right side of work facing you. Anchor the yarn, as described for the mattress stitch. Insert the threaded needle under a stitch (under the 2 loops of a stitch) inside the bound off edge on one side, and then under the corresponding stitch on the other side. Work a few stitches in this manner, then pull the yarn firmly; you don’t have to pull after working each one. You’ll notice that from the right side the stitches are lined up with the appearance of a completed row of knitting.
Invisible Vertical to Horizontal Seam
This seam can be used to sew in the top of a drop shoulder sleeve to an armhole edge, or bound off stitches to rows. Usually there are more rows per inch than stitches, so occasionally pick up two bars on the side with rows for every stitch on the bound off edge. Insert threaded needle under a stitch on bound off edge (vertical piece), followed by one or two stitches of the side with rows (horizontal piece). Try to duplicate the tension of the knit stitch. Continue weaving back and forth until the seam is complete. This method takes a little practice, but is great to use with bulky garments, where a back stitch would make too thick of a seam.
Note: When finished any method of seaming, weave in the yarn end in the seam on the wrong side of the work, and snip close to the fabric.
The above seaming methods are the most common ones used for sewing knit fabrics. The result is a seam that is invisible on the right side, and appears like a continuous piece of knitting. Seaming is an important step in giving your work a professional finish. Practice these methods for flawless results.