Mattress stitch knitting seam
A vertical mattress seam joins the bars between stitch columns with the right sides facing you.
Work the technique
Place pieces side by side. Insert the needle under one bar between the edge and next column on the left, then the matching bar on the right. Continue a few centimetres before pulling the yarn gently to close the gap.
Try it on a sample
Make two 12-stitch stockinette swatches with matching row counts. Join bar for bar and compare the seam length with the pieces.
Check and recover
If lengths differ, distribute the extra rows gradually instead of gathering them at the end. Keep the seam supple and avoid drawing the seam shorter than the fabric.