One of the most common methods of seaming is Mattress Stitch. This is excellent for most sweater seams and leaves a small but comfortable inside seam. It is sometimes called "Invisible Seam" because it almost disappears. Thread a yarn needle with matching yarn. With right sides facing you, place edges to be seamed together.
Beginning at hem edge secure yarn through both sides. With needle pointing up, pick up the bar between first and second stitches on right hand side. Cross to opposite side, pick up bar, return to right hand side. Put needle into the same space it came from and pick up bar. Continue in this manner until four or five stitches have been made. Be sure to match rows. Tighten yarn until it holds seam firmly. Continue stitching picking up every row bar until seam is finished.
When seaming shoulders with mattress stitch, pick up a whole stitch from each side, matching stitches. Then pull firmly to hide seam. Rows should be matched. If extra rows must be "worked in" do this near the armhole, easing gradually. This method may be used to sew in sleeves.
© Reprinted with permission from Craft Yarn Council of America's Certified Instructors Program