Learn to Knit - Mattress Stitch Seam
Mattress stitch is worked with the right side side up and becomes invisible from the right side when finished.
With the right sides facing you, lay the pieces to be sewn flat with edges next to each other, lining up rows and stripes as much as possible. Insert the needle between the first and second stitches on the first row. Slide the needle under two rows, then bring it back to the front between the first and second stitch of the row.
Return to the opposite side and, working under two rows throughout, repeat this zig-zag process, always taking the needle under the strands that correspond exactly to the other side and going into the hole that the last stitch on that side came of, being careful not to miss any rows.
The secret to the Mattress Stitch is to keep the seam elastic without allowing it to stretch too much. The best way to do this is to work very loosely for a few inches, and then pull the sewing yarn so that the stitches pull together. Give the seam a little tug from the top and bottom to add a bit of stretch back in.
If purl rows are on the right side, you may have an easier time working under one row rather than two.
When joining garter stitch, work into the center of the first stitch fo the row instead of between the first and second stitches. When the seam is pulled closed, the garter stitch ridges will line up correctly.
When joining two ribbed sections, work into the center of the first stitch of the row instead of between the first and second stitches. When the seam is pulled closed, one complete knit stitch will be formed and the seam will be nearly invisible.