How to Work a Horizontal Mattress Stitch Seam

When you need to sew knitted pieces together, mattress stitch is the way to go. A mattress stitch seam is simple to work, strong, and nearly invisible. Here’s a step-by-step mattress stitch knitting tutorial to help you master seaming knits.

Above: An in-progress horizontal mattress stitch seam

This tutorial is for seams worked horizontally which join cast-on or bound-off edges together, such as shoulder seams or underarm seams. For seams worked vertically to join side edges (such as a side seams), check out the companion vertical mattress stitch tutorial.

How To Do Horizontal Mattress Stitch Seams

1. Arrange your pieces right side up with the edges to be seamed parallel to each other (Photo 1). These swatches simulate the shoulder of a cardigan, with the back piece on the left and the right front piece on the right. They have been bound off with the Sloped Bind-off technique. The steps are exactly the same for a pieces with a standard straight bind off, or for joining cast-on edges.
2. Thread a blunt tapestry needle with the yarn you’ll use for seaming. I’ve used a contrasting color so the stitches are easy to see. You’ll use your sweater yarn or something in a matching color. Locate the very first stitch in the very first row under the edge. Bring your needle up through the center of that first stitch (Photo 2).
3. Cross to the other piece. Again working in the very first row under the edge, take your needle down to the wrong side through the center of the very first stitch of the row, then back up through the center of the next stitch in the row (Photo 3).
4. Cross back to the first piece. Take your needle down to the wrong side in the same spot where you came up in Step 2, then back up through the center of the next stitch in the row (Photo 4). Note that your needle is always moving toward the end of the seam, parallel to the edges being joined.

5. Cross back to the second piece. Take your needle down to the wrong side in the same spot where the yarn comes out from Step 3, then back up through the center of the next stitch in the row (Photo 5).
6. Continue in this fashion, crossing from one side to the other. As you can see, your seaming stitches follow the path of a new row of knitting between the two garment pieces (Photo 6). I find it easier to see what I’m doing if I leave my stitches fairly loose.
7. Here’s the magic part. When your seam is a couple of inches long, pull firmly on the seaming yarn to tighten those stitches. The seaming yarn will disappear into your piece, leaving you with a smooth, nearly invisible seam (Photo 7). After you’ve pulled the yarn tight, hold both ends of your seam and pull to give it a good stretch. This ensures your seaming yarn isn’t so tight that it puckers the stitches and that your seam won’t break if your sweater is stretched in wearing.
8. Continue until your seam is complete, stopping every few inches to tighten the stitches. (Photo 8). Don’t work more than two or three inches before pulling those stitches tight. Every time you pass the needle through the knit fabric is like another pair of little woolly hands holding on to the seaming yarn. If you wait too long to tighten your stitches, you might have a hard time pulling the yarn through. Your seaming yarn may even break. Remember to give the seam a good stretch after every tightening.

Don’t avoid seamed sweaters because you lack confidence in your seaming skills. With just a little practice, you’ll be a mattress stitch pro and you’ll be making better sweaters.

Sandi Rosner
Technical Content Editor, Yarn

Originally published 4/10/2009. Updated 9/7/2022.

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