Mattress Stitch
The mattress stitch is used for joining two knitted edges together by sewing around the outermost stitches of the respective parts. This technique is perfect for obtaining a very neat and professional looking seam.
In my example, I show you how to sew selvedge stitches together with a cast-on edge. But the principle is always the same and therefore also works with two selvedge stitch pieces.
Place your two edges side by side, with the right sides facing up.
The bottom piece has selvedge stitches on the knit edge. Go around this selvedge stitch with your darning needle.
Now go around the outermost stitch of the upper part.
Tighten the yarn so that the lower selvedge stitch and the upper cast on edge disappear onto the wrong of the seam.
If you want to sew two selvedge stitch edges together, in this step you go around the selvedge stitch of the other piece in the same way as in step 1.
Continue alternating these two steps until you reach the end. Fasten your end on the wrong side and cut it.