The three main seams for joining knitted pieces

The seam you choose depends on where the join sits on the finished garment and how much bulk you can tolerate. Mattress stitch creates an invisible seam and works best for vertical joins like side seams and sleeve caps. Backstitch is stronger and faster but leaves a visible line, so use it for seams that will be hidden inside a garment or for pieces that take heavy stress. Grafting (also called Kitchener stitch) joins two live edges without a visible seam line and produces the smoothest result, but it requires both pieces to have active stitches on the needle.

Each method uses the same basic principle: you're threading yarn through the loops of your knitted fabric to hold two pieces together. The difference is the angle and depth of each pass, which determines whether the seam shows and how strong it becomes.

Key Takeaways

  • Mattress stitch creates an invisible seam by running yarn up the sides of your knitted pieces and is the standard choice for visible seams on finished garments.
  • Backstitch is the fastest method and produces the strongest seam, but it leaves a visible line and works best for hidden or stress-bearing seams.
  • Grafting joins two live edges without any seam line at all, creating a seamless join that looks like one continuous piece of knitting.
  • You need a tapestry needle (blunt tip) and yarn that matches your project weight; using yarn that's too thin will create weak seams, and yarn that's too thick will pucker.
  • Tension matters more than speed — a loose seam will unravel, and a tight seam will pucker the fabric and restrict stretch.

Mattress stitch for invisible side seams

Mattress stitch is the most common choice for joining the sides of sweaters, cardigans, and other garments where the seam will be visible on the finished piece. It creates a seam that sits between the two edges and is nearly invisible from the outside.

To work mattress stitch, lay your two pieces flat with the right sides facing up and the edges you want to join touching but not overlapping. Thread your tapestry needle with a length of yarn about an arm's length long. Start at the bottom of one piece by bringing the needle up through the first stitch on the edge, then across to the other piece and up through the first stitch there. Now work upward: insert the needle horizontally under two horizontal bars (the bumps between stitches) on one side, then cross over and insert under two bars on the other side. Keep the tension even — not so tight that the fabric puckers, but tight enough that no gap shows between the pieces. After every inch or so, pull the yarn snug to close the seam.

The key is working under the horizontal bars, not through the stitch loops themselves. If you go through the loops, you'll create a visible line. The bars sit between the columns of stitches and are invisible from the outside, so your seam hides inside the fabric.

Backstitch for strong, fast seams

Backstitch is faster than mattress stitch and creates a stronger seam, making it the right choice for seams that will be hidden (like the underside of a hem) or for pieces that need to withstand stress (like the bottom of a bag or the crotch of pants).

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 both layers one stitch in from the edge. Push the needle back down through both layers one stitch to the left, then bring it back up one stitch to the right of where you came out. Continue this pattern, always going back one stitch and forward one stitch. Keep your stitches small and even — about the same size as the knitted stitches themselves.

Backstitch creates a visible seam line because you're working on the inside of the fabric, and that line will show when you turn the piece right-side out. The seam is also thicker than mattress stitch because you're going through multiple layers. This thickness is an advantage when you need durability, but a disadvantage when you want a smooth drape.

Grafting for seamless joins

Grafting (Kitchener stitch) is the method to use when you want no seam at all — the two pieces will look like they were knitted as one. It only works when both pieces have live stitches still on the needle, so you don't cast off either edge before grafting.

Arrange the two pieces so the needles are parallel and the stitches face the same direction. Thread your tapestry needle with a length of yarn. Bring the needle through the first stitch on the front needle from right to left, then through the first stitch on the back needle from left to right. Now work in a pattern: go through the front stitch again from left to right (removing it from the needle), then through the next front stitch from right to left. Cross to the back needle, go through the stitch you just entered from left to right (removing it), then through the next stitch from right to left. Repeat this pattern all the way across.

Grafting takes practice because the rhythm is specific and straightforward to lose. The result, though, is worth the effort — the join is completely invisible and has the same stretch and drape as the rest of the fabric. This is the standard method for closing the top of a hat, joining the sides of a seamless sweater, or attaching a sleeve cap when you want a professional finish.

Choosing the right yarn and needle

Use a tapestry needle (also called a yarn needle) with a blunt tip, not a sharp sewing needle. The blunt tip slides between the loops of your knitting without splitting the yarn fibers. The eye should be large enough that your yarn threads easily without fraying.

Match your seaming yarn to your project yarn in both weight and fiber content. If your project is worsted weight, use worsted weight yarn to sew the seams. If you use yarn that's too thin, the seam will be weak and may unravel. If you use yarn that's too thick, it will create lumps and pucker the fabric. The best practice is to use a length of the same yarn you knitted with, or a yarn that matches it closely in thickness.

Cut lengths about an arm's length long (roughly 18 to 24 inches). Longer lengths are more likely to fray and tangle as you work. You can always thread the needle again when you run out.

Tension and common mistakes

The most common mistake is pulling the seaming yarn too tight. A tight seam will pucker the fabric, restrict its stretch, and make the edges curl. A loose seam will gap and eventually unravel. The right tension is snug enough that no gap shows between the pieces, but loose enough that the fabric lies flat and moves freely.

The second mistake is working too fast. Seaming is slower than knitting, and rushing leads to uneven stitches, missed loops, and seams that look lumpy. Take your time and check your work every few inches by looking at the seam from both sides.

If you're working mattress stitch and the seam starts to look thick or visible, you may be going through the stitch loops instead of under the horizontal bars. Stop, look at where your needle is going, and adjust. If you're working backstitch and the seam is puckering, loosen your tension slightly on the next few stitches.

When to use each seam on a real garment

On a typical pullover sweater, use mattress stitch for the side seams and sleeve seams — these are visible and need to look neat. Use backstitch or mattress stitch for the shoulder seam, depending on how much bulk you can tolerate at the neckline. If you're grafting the sleeves into the armholes, use grafting for a seamless cap.

On a cardigan, use mattress stitch for the side seams and sleeve seams. Use backstitch for the button bands and front edges, since these seams will be hidden under the band or facing. Use mattress stitch for the back neck seam if you're joining pieces, or graft if you have live stitches.

On a hat, use grafting to close the top if you have live stitches, or mattress stitch if you've cast off. Use mattress stitch for the back seam. On a bag or other item that takes stress, use backstitch for durability even if it means the seam will be slightly visible.

Frequently Asked Questions

Can I use regular sewing thread instead of yarn to sew knitted seams?

No. Sewing thread is too thin and will break under the stress of knitted fabric, which stretches and moves. Use yarn that matches your project in weight. If you don't have leftover yarn, buy a small amount of yarn in the same weight and fiber type.

What if I sew the seam and it looks lumpy or uneven?

You can rip it out and try again. Use a seam ripper or small scissors to cut the seaming yarn every few stitches, then pull it out. This won't damage your knitted pieces. Rip back to where the seam starts to look good, then continue from there with fresh yarn.

Do I have to block my pieces before seaming them?

Blocking before seaming makes the edges more even and easier to work with, especially if your pieces have curled edges or uneven tension. It's not required, but it produces a neater result. If you don't block, take extra care to keep your seaming tension even so the pieces don't pucker.

How do I hide the yarn tail after I finish seaming?

Thread the tail through your tapestry needle and run it along the inside of the seam for an inch or two, weaving it through the seam stitches. Then bring the needle out and trim the tail close to the fabric. The tail will stay hidden inside the seam.

Can I use mattress stitch on ribbed edges?

Yes, but it's trickier because the ribs create an uneven edge. Work under the horizontal bars just as you would on stockinette, but be careful to keep your tension even since the ribs may pull differently than flat fabric. Backstitch is often easier on ribbed edges because you can work through the stitch loops more consistently.