Mattress stitch is used to sew together the sides of two stockinette stitch pieces of knitting. It creates an invisible seam and is often used to make up garments that are knitted in pieces.
The most important factor in this technique is identifying the “ladder rungs” or horizontal bars of yarn. These are hiding behind the Vs of your stitches. So, before you begin to sew, gently pull your knitting until you see those “ladder rungs” they should look like the photo above.
Decide Where to Begin
There are “ladder rungs” between every stitch and behind every stitch, there are even some right on the edge of the knitting. Count in at least one stitch from the edge of both of the pieces you are seaming. You are looking for a column of stitches, close to the edge where the stitching is even.
Lay the two pieces to be seamed side by side, with right side facing. Then secure the sewing yarn (shown above in red) in the bottom left corner of the right hand piece. Sew from back to front through the bottom stitch in the column you will be seaming. Sew through the same stitch again in the same direction to secure.
Begin the Seam
Keep the two pieces side by side with the right side facing you. Sew from front to back, behind two “ladder rungs” and then back to front on the left hand piece. Return to the right hand piece, sew through the same hole as you attached the yarn, go behind two “ladder rungs” and from back to front. As shown in the photo above.
Sew Through the Same Hole
You should pull the yarn tight as you go along, however the photo above shows how the stitches look if you don’t pull the yarn tight. As you can see you are sewing into the same hole as you have previously sewn out from.
Pull Tight as you Sew
Here’s the magic part! When you have made a few stitches, pull the sewing yarn to close the seam. It will look like the photo below.
As you can see, the red sewing yarn is now invisible and the seam appears to be made of two halves of a column of V stitches.