The three main ways to join knitted pieces
You join two finished pieces of knitting by sewing them together with yarn and a blunt needle. The three methods you will encounter are the mattress stitch (which creates an invisible seam and is the most common choice), the backstitch (which is fast and visible, often used on edges that will be hidden), and the grafting stitch (which joins two live edges—stitches still on needles—so there is no seam ridge at all).
Which method you use depends on where the seam will be, whether you want it to show, and whether your stitches are still active or already bound off. Mattress stitch works on bound-off edges and is nearly invisible from the outside. Backstitch is the quickest but leaves a visible line. Grafting is the most seamless but only works when both edges have live stitches.
Before you start any seam, weave in the loose ends at each piece's corners so they do not get caught in your stitches. Use a length of the same yarn you knitted with, or a matching yarn if the original is too thick. A blunt tapestry needle prevents you from splitting the yarn strands as you work.
Key Takeaways
- Mattress stitch creates an invisible seam by running your needle through the loops between edge stitches on two pieces, and is the standard choice for joining sweater pieces.
- Backstitch is a fast visible seam that works on any bound-off edges and is often used on seams that will be hidden inside a garment.
- Grafting joins two pieces with live stitches so there is no seam ridge, but requires both edges to still be on needles or held on waste yarn.
- The yarn you use for seaming should match the weight and color of your knitted piece, and a blunt needle prevents you from splitting yarn as you work.
- Pinning or basting your pieces together before seaming helps you keep edges aligned and catch mistakes before they are sewn in permanently.
Mattress stitch: the invisible seam
Mattress stitch is the most common seaming method because the finished seam is nearly invisible from the outside of the garment. It works by running your needle under the horizontal bars of yarn that sit between your edge stitches, rather than going through the stitches themselves. This creates a seam that sits inside the edge rather than on top of it.
To work mattress stitch, lay your two pieces flat with the right sides facing you and the edges you want to join side by side. Thread your blunt needle with a length of yarn about 18 to 24 inches long. Start by bringing your needle up through the first stitch on one piece, then across to the other piece. Insert your needle under the horizontal bar between the first and second stitch on that piece, then come back out under the next bar. Return to the first piece and repeat, going under the bars in the same way. After every few stitches, gently pull the yarn snug—not tight—and the seam will fold inward and disappear.
The key to a neat mattress stitch seam is consistency: work under the same bar on each piece every time, and pull the yarn with even tension. If you pull too hard, the seam will pucker. If you do not pull hard enough, it will be loose and floppy. Practice on a swatch first if you are new to this stitch.
Backstitch: the fast and visible seam
Backstitch is the quickest seaming method and works on any bound-off edges. Unlike mattress stitch, backstitch goes through the actual stitches themselves, so it creates a visible line on the inside of the seam. This makes it ideal for seams that will be hidden—the inside of a sleeve, the underside of a collar, or anywhere the seam will not be seen from the outside.
To work backstitch, place your two pieces with right sides together (the sides you want to show on the outside facing each other). Thread your needle and bring it up through the first stitch on both pieces at once. Then insert the needle back down through the stitch you just came out of, and bring it up two stitches ahead. This creates a line of overlapping stitches that hold the pieces firmly together. Continue along the entire edge, keeping your stitches small and even.
Backstitch is stronger than mattress stitch and is often used on seams that will bear weight or stress, such as the sides of a bag or the underarms of a sweater. Because the seam is visible on the inside, it does not matter if your stitches are slightly uneven—they will not show from the outside.
Grafting: joining live stitches with no seam
Grafting, also called Kitchener stitch, joins two pieces of knitting so perfectly that there is no seam at all—the stitches from one piece flow directly into the stitches of the other. This works only when both edges have live stitches (stitches still on needles or held on waste yarn), so you must plan ahead and not bind off those edges.
To graft, hold both needles parallel with the right sides of the knitting facing you. Thread your blunt needle with a length of yarn. The pattern is: go through the first stitch on the front needle as if to purl, then through the first stitch on the back needle as if to knit, then back through the first stitch on the front needle as if to knit, then through the next stitch on the front needle as if to purl. Repeat this pattern across, always going through two stitches on the front needle before moving to the back needle. After every few stitches, gently pull the yarn snug so the seam closes and the stitches blend together.
Grafting takes practice and concentration, but the result is a seam that is completely invisible and has no ridge. It is often used on the toes of socks, the tops of hats, and anywhere a seamless finish is important.
Preparing your pieces before seaming
Before you start any seam, make sure both pieces are the same size and shape. If one piece is slightly larger, you can gently block it (wet it, pin it to the correct dimensions, and let it dry) to match the other. Weave in all loose ends at the corners and edges so they do not get caught in your seaming yarn.
Pin your pieces together along the seam line using stitch markers or safety pins to hold them in place. This prevents the edges from shifting as you work and helps you catch mistakes before they are sewn in. If you are nervous about a seam, you can baste it first with a contrasting yarn in a straightforward running stitch, then go back and seam it properly once you are confident the pieces are aligned.
Use a length of yarn about 18 to 24 inches long for each seam. If you run out of yarn before the seam is finished, weave in the end and start a new length, overlapping the old end by an inch or so. This prevents the seam from coming apart if one length of yarn breaks.
Seaming different stitch patterns and edges
Mattress stitch works best on stockinette stitch (the smooth V-pattern side of knitting) because the horizontal bars are clear and straightforward to see. On textured stitches like garter stitch or ribbing, the bars are less obvious, so you may need to work more slowly and use a magnifying glass if your eyesight is not sharp.
If you are seaming a ribbed edge to a stockinette edge—for example, joining a ribbed cuff to a sleeve—use mattress stitch on the stockinette side and backstitch on the ribbed side. This gives you the invisible seam where it shows and a strong seam where the stitches are more delicate.
When seaming curved edges, such as armholes or necklines, you may need to ease one piece slightly to fit the other. Work your seam stitch by stitch rather than trying to match row for row, and adjust the tension as you go so the curve lies flat.
Troubleshooting common seaming problems
If your seam is puckering, you are pulling the yarn too tight. Loosen your tension and try again on the next section. If your seam is loose and floppy, you are not pulling hard enough. Increase the tension slightly, but not so much that the knitting bunches.
If you notice a gap between the two pieces as you seam, you may have skipped a stitch or worked through the wrong bar. Carefully undo the last few stitches, find where the gap is, and start again from that point. It is better to redo a small section than to leave a hole in your finished garment.
If the seam is visible from the outside when it should be invisible, you may have worked backstitch instead of mattress stitch, or you may have worked mattress stitch through the wrong part of the stitch. Check your technique and redo the seam if necessary.
Frequently Asked Questions
Can I use a regular sewing needle instead of a tapestry needle?
A regular sewing needle is too sharp and will split your yarn strands, making the seam weak and uneven. A blunt tapestry needle slides between the yarn fibers without breaking them. If you do not have a tapestry needle, a yarn needle (which is the same thing) costs a few dollars and is worth the investment.
What if my two pieces are slightly different sizes?
If one piece is a little larger, you can block it to match the other before seaming. If the difference is more than a quarter inch, you may need to redo one piece. When seaming, you can ease the larger piece slightly by taking slightly smaller stitches on that side, but this only works for small differences.
How do I join pieces in the round, like for a seamless sweater?
If you are joining pieces that form a tube (like sleeves to a body), use mattress stitch on the side seams and grafting or a three-needle bind-off on the underarm seams where the stitches meet at right angles. The three-needle bind-off is faster than grafting and creates a sturdy seam.
Can I machine-wash a sweater with hand-sewn seams?
Hand-sewn seams are generally strong enough for machine washing on a gentle cycle, but seaming by hand gives you more control over tension and creates a more durable seam than machine sewing. Wash in cold water and lay flat to dry to avoid putting stress on the seams.
What yarn should I use for seaming if the original yarn is too thick?
Use a yarn that matches the weight and color of your knitted piece as closely as possible. If the original yarn is very bulky, a worsted-weight yarn in the same fiber and color will blend in and seam more easily. Test the seam on a swatch first to make sure the color match is acceptable.