Row to Row Seaming
Often called the mattress stitch, this is a great technique for joining the front and back of a sweater at the sides, or two pieces that are joined along the row edge.
Lay the two pieces to be joined (piece A & piece B) edge to edge and line up the stitches so that each row on piece A corresponds to a row on piece B.
Step 1: Using a darning needle pull the yarn from the back of piece A through the center of the first stitch in the row at the edge of the piece.
Step 2: On piece B, insert the needle into the center of the first stitch in the first row and pull the needle out from the center of the first stitch in the second row.
Step 3: Return the needle to the piece A and insert into the center of the first stitch in the second row, echoing what you have just done on piece B.
Repeat the above steps, working all the way down the edge of the pieces until the two pieces are joined.