Mattress stitch is used to invisibly seam two vertical pieces of stocking stitch. Take a threaded darning needle and pass it up and under each corner stitch to secure the yarn in a figure-8 motion. The seam is joined by picking up and sewing the horizontal bars between two columns of V stitches. Starting on the left side insert the needle under the first two bars and draw the yarn through. Find the
adjacent stitches on the right hand side and do the same. Continue zig-zagging up the piece, making sure to always sew stitches from the same column on each side. Pull the working yarn every few centimetres to zip it closed.