The Mattress Stitch is a finishing technique for vertical seaming. It creates an invisible join between pieces worked in stockinette stitch or ribbing, perfect for so many things, such as sewing the front and back of a sweater together. For demonstration purposes, I have used a different yarn in a contrasting color, but typically you would use the same yarn as your knit pieces.
How to Join in the Round Invisibly To join in the round invisibly, cast on one more stitch than your pattern calls for. Then, when you’re ready to join in the round, slip this extra stitch to the left needle. Using the working yarn and the yarn tail held together, knit the first two stitches together. Now drop the tail and knit as you normally would.