British moss stitch, also known as seed stitch, is knitted over an even number of stitches to create a textured, reversible fabric. It differs from American moss stitch in that it alternates the knit and purl stitches in every other row instead of every two rows. Unlike stocking stitch, which tends to curl at the side edges, moss stitch lies flat and makes a firm, springy fabric. To avoid thick, lumpy seams join moss stitch pieces with a flat seam, using the almost invisible mattress stitch.
Thread the needle with a single strand of the yarn approximately 18 inches in length. Don't use a yarn in a thicker gauge than your knitted pieces.
Lay the pieces to be joined flat on a table or other surface with the right sides of the knitting facing upwards. Butt together the two edges to be joined and place a few pins horizontally across the two knitted pieces to keep them lined up.
Insert the needle into the bottom corner of the first piece of knitting, one full stitch in from the vertical edge. Pull the yarn through, leaving a 2- or 3-inch tail.
Insert the needle vertically into the equivalent stitch on the second piece. Pull the yarn through but don't close the edges together completely.
Continue inserting the needle into first one piece and then the other, passing it under the bottom bar between the first and second stitches from the edge each time. Make sure the pieces remain lined up throughout until you've sewn the length of the seam.
Hold both ends of the yarn and pull them gently so your stitches close up and the two edges come together. Be careful not to break the yarn, nor to pull it so tightly that the edges pucker.
Fasten off the yarn at the beginning and end of the seam by weaving in the ends on the wrong side.