Invisible stitching or Mattress Stitch is a finishing technique for vertical seams on knitted pieces.
This technique creates an invisible seam between pieces knitted in stockinette stitch, garter stitch, or ribbing, which is perfect for many things, such as sewing the front and back of a sweater together or two squares to make a blanket.
How to sew two knitted pieces together?
Invisible sewing step by step
First, align the two pieces you want to sew. Bring your threaded tapestry needle from the back of your work to the front, where you want to start your stitching.
For blindstitching, you sew two pieces of knit together by passing a strand of yarn through each side, alternating back and forth, working vertically along the edges. The strand of yarn you're grasping is a horizontal bar that runs between the stockinette stitches (or "Vs"). If you gently pull on the right and left edges of your knit, you can see these horizontal bars (or "ladders") between the "Vs."
Pass the needle between the bars of the first V from the back of the knitting to the front.
Then pass the needle to the level of the bar of the V by passing under the bar located in the center of the V.
Continue working vertically along both edges, passing the tapestry needle under each bar of each piece.
To close the invisible seam, gently pull the length of thread from the top or bottom. If you are working on a long seam, it is best to sew a few inches, then pull to close, sew a few inches again, and pull, continuing in this manner until the end of the seam, making sure to maintain constant tension throughout.
If you want to sew two pieces in garter stitch, you will proceed in the same way, except that you will not pass your needle between the bars of the V but rather the rebound of the knit stitch by passing the needle from the bottom to the top.
Illustrations: © Sylvain Daudier
