A step-by-step tutorial on joining two knitted pieces with the mattress stitch creating an invisible seam
If you want to join two knitted pieces together, then there’s no way around learning the mattress stitch. This vertical seaming technique creates an invisible join between two pieces worked in stockinette stitch or other stitch patterns with a knit stitch edge.
Typically, you will use it to join the back and front of a sweater together. But you can also use it to join a flat project so it appears as if you knit it in the round. The mattress stitch will create a noticeable ridge on the inside/wrong side of your work, though.
Note: I am using contrasting yarn for demonstration purposes only. And I earn a small commission for purchases made through links in this article
Let’s dive right into it, eh?
Tip: Check out my tutorial on seaming garter stitch with mattress stitch as well (the technique is similar but different).
- Works with every yarn. I am using the Schachenmayr Catania Grande in this tutorial
- A (cast on/cast off) tail long enough for seaming or an extra seaming yarn in a similar color (recommended for structurally weak knitting yarns like Cashmere, etc.)
Thread the tail or a separate piece of seaming yarn onto a blunt tapestry needle (also known as a darning needle). You will need approximately twice as much yarn as the final length of the seam. Then lay the two knitted pieces you want to join side by side with the knit side (right side) facing towards you. Make sure one of them is not upside down.
Note: Consider blocking your project before you start with the seaming.
- Identify the first column of stitches on the right piece and go underneath the first bar between two knit stitches
You can start seaming on either side. I always start on the right. Either way, find the very first column of knit stitches and go in between them with your tapestry needle. There should be a little bar in between the "V"s of two adjacent knit stitches. It looks a bit like a little ladder. Go underneath the very first bar and pull your tapestry needle through.
Note: If you are working with a separate piece of seaming yarn, leave a tail of 5-7 inches for weaving in the end later.
- Go underneath the first bar between two knit stitches on the left side
Next, find the first column of stitches on the piece as well. Then draw your needle underneath the bar (bottom to top) between the first two stitches as well.
- Draw the yarn through the next bar on the right piece
Go back to the first piece and find the bar directly above the one you already went through, and draw your tapestry needle through again.
Note: You can also go through two bars at the same time depending on your personal preferences and the density of your fabric. For very fine knitting (say 3.00mm needles and below) you will not notice a difference. But you have to decide on one way to get an even and invisible seam.
- Draw the yarn through the next bar on the left side
Next, go back to the left side and draw the yarn underneath the second bar as well.
- Repeat steps 4+5 and tug on the yarn every couple of stitches
Continue going back and forth between the two knitted pieces picking up the bars with your tapestry evenly on both sides. Tug on the yarn every inch or so stitches to close the seam. Don't pull too tight. Otherwise, your seam will buckle! If it does, stretch it out gently before you continue seaming.
- Weave in the ends
Once you are finished seaming, give the tail(s) a gentle tug. The fabric shouldn't buckle, but there shouldn't be any excess hidden in between either. If you pulled too tight, you can gently massage the seam into place again. And then, pick up the tapestry needle and weave in the tails along the seam on the backside.
The finished mattress stitch join will be virtually invisible from the right side. It will appear like one continuous line of stockinette stitch.
There is, however, a little seam on the wrong side. This seam will become bigger if you don't connect the first two columns of knit stitches but the second or third column.
While I normally don't recommend doing this, you can file it as a last resort to fix the size of a too-large sweater. Also, some stitch patterns don't result in a nice edge and it might be better to pick the second column.
Also, there are bars in between the knit stitch V's and between two knit stitches. It doesn't actually matter which bar you pick as long as you pick the same kind of bar on both sides. And actually, if you choose different bars on either side the sea will still be almost invisible (see picture right above).
The finished mattress stitch join will be virtually invisible from the right side. It will appear like one continous line of stockinette stitch.
There is, however, a little seam on the wrong side. This seam will become bigger if you don’t connect the first two columns of knit stitches but the second or third column.
While I normally don’t recommend doing this, you can file it as a last resort to fix the size of a too-large sweater. Also, some stitch patterns don’t result in a nice edge and it might be better to pick the second column.
Also, there are bars in between the knit stitch V’s and between two knit stitches. It doesn’t actually matter which bar you pick as long as you pick the same kind of bar on both sides. And actually, if you choose different bars on either side the sea will still be almost invisible (see picture right below).
Tips & tricks for a better mattress stitch
This seaming method is pretty easy and straightforward. However, it’s certainly much easier to join two pieces in plain stockinette stitch than it is to join more complicated patterns or challenging yarns. Here are some tips for you:
1. Hold the seam between your fingers
I usually hold the two pieces between my fingers (see picture above). This makes it much easier and faster to join them with a mattress stitch. That way, you can also bring it a bit closer to your face, which makes finding the next bar you need to go through a bit easier as well.
2. Use pins to lock the seam
Some yarns are very fuzzy. Also, if you are holding two threads of yarn at the same time (like a nice wool base and some mohair yarn to add a nice halo), it can be extremely difficult to identify the next bar. If there are some increases in between, it will be even more difficult.
That’s why you can pin the seam in place. If you skip one bar it probably won’t be noticeable in the end. If there is a bigger imbalance, it will make your seam pucker. But if you pre-pinned the seam, you will a) notice if you did so and b) be able to adjust for it.
3. Add a selvage stitch for Fair Isle
Fair Isle or Stranded knitting usually doesn’t leave the neatest edge. Or rather, not an edge where you can easily identify the bars between a V. And even if you can, you often end up pulling on a float or other things that might mess up your seam.
That’s why adding two selvage stitches can be really helpful. Half of that selvage stitch won’t be visible after seaming anyway, so it’s less of a problem than you might think.
Note: If you have ribbing or other stitch patterns with a purl stitch in between, the same applies.
4. Be very careful when unraveling a Mattress seam
Sometimes, even if you take extra care, you end up skipping a couple of bars and your seam puckers. One thing you really should be aware of is that all that going in and out puts a lot of stress on the yarn. If you reverse the seam one stitch at a time, then you will have to pull the entire length of yarn through each stitch again. This creates a lot of friction and it will do two things:
- it will slowly grind away the substance of your seaming yarn (it will untwist and maybe even tear apart). Think of it like this: Every time the yarn passes through it takes away a single little hair. There are hundreds of them in a single strand of yarn. But if you go through it a hundred times, none would be left. If you were to use it again, you might create a structurally weak seam.
- And could possibly weaken the stitches at the edge themselves in a similar way.
Instead, simply pull out the whole seam. Don’t try to reverse-engineer it. Just tug on it until the whole seam comes undone in one pull.
If that’s not possible (because you didn’t use a separate seaming yarn or the seam is too long), then cut the yarn off ever so often so you don’t have to drag the whole length of it through each stitch.
5. Don’t (or do) weave in the tails through the ridge on the backside
You should be aware of one fact: The seam you create with a mattress stitch has not a lot of give – unlike your stockinette stitch. One thing I often see is that people weave in the end(s) by going through the ridge of the seam. This will further reinforce it and take away even more elasticity.
Depending on the place of your seam this may or may not be what you want. If it isn’t simple weave in the tail on the very edges of the seam without going all the way through to the other side.