When I'm seaming a sweater, one of the most used stitches I use is the Mattress stitch. Also known as the Ladder Stitch, I love this stitch because when done right, it's nearly invisible, adds very little bulk to the seams, is quick to do, easy to take out, and practically perfect. Let me tell you how I do the mattress stitch.
2 quick notes: here I start in the middle of a project. Normally I start at the beginning of the seam and work my way up, but that's hard to photograph well. Also, I'm using embroidery floss in the tutorial, both because it's in a contrast color, and because it's cotton, and easy to pull out. Most people like seaming with the yarn they used in the project, but if it's too delicate, or too bulky, embroidery floss in a similar color works beautifully. Mattress stitch done right should be practically invisible.
The first part of learning the mattress stitch involves taking a good look at your knitting. Look between two stitches. See the horizontal bars that run between the stitches? This is where the sewing action will take place. You'll want to be working the mattress stitch between the 1st and 2nd columns of stitches on the edge of your work. See Below?
Right, so we now know where the stitches are supposed to go. When I teach the matress stitch, I always say to go beneath 2 horizontal bars each time. Some directions will tell you to go underneath only one bar, but when you're first learning, it's easier to go underneath 2. I normally go underneath 2 bars if the yarn is an aran weight yarn or lighter, just because mattress stitch goes a lot more slowly when only going underneath 1 bar. For aran weight yarns and higher (like bulky) I'll go underneath only one bar, because each "stitch" is so big. So in most instances, just go underneath 2 bars between the stitches. I like to start with the right side, just because I'm right handed.
Now, what happens if you are working columns of stitches to rows of stitches? You still use the mattress seam, the placement on the row side is just a little different. Take a look at the edge of your knitting, where you bound off or cast on. See how the v's connect to make a zig-zag? This is where we'll be working. Let's look at an individual stitch. Here, I've got it lifted up by my needle.
It's important to note, you can switch between working columns to columns, columns to rows, rows to rows, and back again. You can be seaming on the stockinette side, and then switch to seaming on the row side.
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