The three ways to join knitted pieces
When you finish knitting separate pieces—a front, back, and sleeves for a sweater, or panels for a blanket—you join them by sewing, not by knitting. The most common method is the mattress stitch, which creates an invisible seam and works on most knit fabrics. Two alternatives are the backstitch, which is faster but visible, and the grafting stitch (also called Kitchener stitch), which joins live stitches and leaves no seam at all.
Which method you use depends on where the seam will be, how much time you have, and whether you want the join to show. Mattress stitch is the default for side seams and shoulder seams on garments. Backstitch works well for seams you'll hide inside a folded edge. Grafting is best for joining the top of a sweater front to the back without a ridge.
Key Takeaways
- Mattress stitch creates an invisible seam by catching the horizontal bars between stitches on each side, and works on most knit fabrics without special preparation.
- You need a tapestry needle (blunt, with a large eye) and yarn that matches your knitting—use the same yarn you knitted with if you have leftovers, or a similar weight and color.
- Backstitch is faster than mattress stitch but creates a visible ridge, so use it for seams that will be hidden or for projects where the seam is a design feature.
- Grafting joins two pieces of live stitches without any seam at all, but requires both pieces to have the same number of stitches and takes practice to execute smoothly.
- Always leave a long tail of yarn when you cast off or bind off your pieces—at least 12 to 18 inches—so you have enough length to sew without running out.
Mattress stitch: the invisible seam
Mattress stitch is the most popular seaming method because the seam disappears into the fabric. It works by catching the horizontal bars of yarn that sit between the edge stitch and the next stitch in from the edge. You work from the right side of the fabric, so you can see exactly what you are doing and catch the correct bars.
Thread your tapestry needle with a length of yarn about 3 feet long. Insert the needle under the horizontal bar on one piece, then under the corresponding bar on the other piece, moving up one row at a time. After every few rows, pull the yarn snug—not tight enough to pucker, but tight enough that the seam closes. The edges will curl inward and hide the yarn, leaving a flat seam that looks like it was knitted as one piece.
Mattress stitch works best on stockinette stitch (the smooth V-pattern side) and on pieces with straight edges. If your pieces have a lot of shaping or if you are joining ribbed edges, you may need to adjust which bars you catch, or switch to backstitch instead.
Backstitch: the fast, visible seam
Backstitch is faster than mattress stitch and works on any stitch pattern, but it creates a visible ridge on the inside of the seam. Use it when the seam will be hidden—inside a folded hem, or on the inside of a garment—or when you want the seam to be a design feature on the outside.
Place the two pieces with their right sides facing each other (the sides you want to show on the finished piece touching). Insert your needle through both layers, one stitch in from the edge, and pull the yarn through. Move forward one stitch, then insert the needle back into the hole you just came out of, and pull through. Move forward again and repeat. You are essentially drawing a line of stitches along the edge.
Backstitch seams are stronger than mattress stitch seams and hold up well on garments that get a lot of wear. The trade-off is that you cannot see the seam from the right side, so you have to be careful to keep your stitches even and your line straight.
Grafting: joining without a seam
Grafting (Kitchener stitch) joins two pieces of live stitches—stitches that are still on the needle—without creating any seam at all. The result looks like you knitted the two pieces as one continuous piece. This method is ideal for joining the front and back of a sweater at the shoulders, or for closing the top of a hat.
Both pieces must have the same number of live stitches. Hold the two needles parallel, with the working yarn attached to the right needle. Insert your tapestry needle through the first stitch on the front needle as if to purl (from right to left), then through the first stitch on the back needle as if to knit (from left to right). Drop the stitch from the front needle. Move to the next stitch on the front needle and repeat, always going through the back needle stitches as if to knit and the front needle stitches as if to purl.
Grafting takes practice, and the rhythm is straightforward to lose. If you drop a stitch or lose your place, you can unravel back a few stitches and start that section again. Many knitters find it helpful to practice on a small swatch before grafting a seam on a finished garment.
Choosing and preparing your yarn
Use yarn that matches the yarn you knitted with. If you have leftover yarn from your project, that is ideal. If not, buy a small amount of the same weight and color—most yarn shops will sell you a single ball even if the pattern calls for more. Yarn that is too thin will break under tension; yarn that is too thick will create a bulky, visible seam.
Cut a length of yarn 3 to 4 feet long for mattress stitch or backstitch, or about 2 feet for grafting. Thread it through a tapestry needle, which has a blunt tip and a large eye. A blunt needle will not split the yarn or poke through the fabric accidentally. If you run out of yarn before the seam is done, weave in the end, tie a new length to it with a square knot, and continue.
Seaming step by step: mattress stitch on a side seam
This is the most common seaming task: joining the right side of the front piece to the left side of the back piece on a sweater or cardigan.
- Lay the two pieces flat with the right sides facing up (the sides you want to show).
- Thread your needle with 3 to 4 feet of matching yarn.
- Insert the needle under the horizontal bar between the first and second stitch on the right edge of the front piece, from the bottom up.
- Insert the needle under the corresponding bar on the back piece, from the bottom up.
- Go back to the front piece and insert under the next bar up, from the bottom up.
- Go to the back piece and insert under the next bar up, from the bottom up.
- After every 4 to 6 rows, pull the yarn snug to close the seam.
- Continue until you reach the top of the seam.
- Weave in the end by threading it through nearby stitches on the inside, then trim.
Common mistakes and how to fix them
The most common mistake is catching the wrong bar—the edge stitch itself instead of the bar next to it. This creates a seam that is too tight or pulls the fabric out of shape. If this happens, unravel back a few rows and start again, being careful to catch the bar one stitch in from the edge.
Another mistake is pulling the yarn too tight. A seam that is pulled tight will pucker and distort the fabric. Pull just enough to close the gap between the pieces; the edges should curl inward naturally. If you have already pulled too tight, you can loosen it by gently stretching the seam with your fingers.
If you are grafting and lose your place in the rhythm, unravel back to where you are confident you are correct, and start again from there. It is better to redo a few stitches than to graft the whole seam incorrectly and have to pull it all out.
When to call in help
If you have never seamed before, it is worth asking an experienced knitter to show you the motion in person. The rhythm of mattress stitch is hard to learn from written instructions alone, and watching someone do it once often makes it click. Many yarn shops offer short classes or will help you get your free guide for free.
If a seam has gone wrong and you cannot figure out where, or if you have already unraveled and reseamed the same spot three times, it is fine to take the piece to a yarn shop and ask for help. A few minutes of hands-on guidance will save you hours of frustration.
Frequently Asked Questions
Can I use the same yarn I knitted with, or do I need special seaming yarn?
Use the same yarn you knitted with if you have leftovers. Seaming yarn is just regular yarn in a thin weight, sold in small amounts. Any yarn that matches your project in weight and color will work fine.
What if my pieces have different numbers of rows?
Mattress stitch can handle small differences—one or two rows—because you can skip a row on the longer piece without the seam looking uneven. If the difference is more than a few rows, you may need to block the pieces to even them out, or use backstitch instead, which is more forgiving of uneven edges.
Do I have to leave a long tail when I bind off?
Yes. You need at least 12 to 18 inches of yarn to seam a typical side seam. If you bind off without leaving a tail, you will have to tie on a new piece of yarn and weave in an extra end. It is much easier to plan ahead and leave the tail.
Can I machine-wash a sweater with seams I sewed by hand?
Yes, as long as the seams are find. Mattress stitch and backstitch seams are strong and will hold up to washing and wearing. Grafted seams are also durable. Use a gentle cycle and lay the sweater flat to dry to avoid stretching the seams.
What is the difference between mattress stitch and backstitch in terms of strength?
Backstitch creates a stronger seam because the yarn passes through both layers multiple times. Mattress stitch is slightly less strong but creates an invisible seam. For garments, both are strong enough. Use backstitch for items that get heavy wear, like the seams on a bag.