I think when so many people said they were interested in information on finishing, what they really wanted to know, at least in part, was how to seam together knit pieces so they look their best. And if you’re working with pieces knit in stockinette stitch — or with selvedge stitches so the seaming is done over stockinette — you’re talking about mattress stitch.
Mattress stitch is something that seems (and I have to be honest and tell you I typed “seams” there first) mysterious and complicated before you do it, but once you understand how to do it and where you need to take your stitches, it’s actually pretty easy.
If you kind of pull on the edge of the knitting you’ll see the ladders between the stitches, and that is where you need to work. Just pick up a ladder on one piece, then the other. Don’t worry about pulling it tight in the beginning, just work an inch or so of stitches, then pull snugly enough so that the seam closes up and makes the knitting look like one continuous piece.
Getting your tension right is key because you want it to look seamless on the outside: not too tight or too loose. I like to pull pretty tight on the yarn, then pull the knitting along the seam to make sure that the seam doesn’t feel too firm.
Another important thing to keep in mind is that you’re always stitching up the same column of ladders. Some people like to seam a half stitch in, some a whole (a whole stitch is a lot easier, to me) and it doesn’t really matter so long as you do it consistently.
My tutorial on mattress stitch is linked above, but there’s also a good video from Knitting Help that may, well, help. The article on mattress stitch from Knitty includes how to do it on garter stitch, which I cover in a separate tutorial.
Have you mastered mattress stitch? I’d love to hear how you like it or if you have any tips to share.