The three seams you'll actually use

Most knitted pieces need seaming when you're done knitting—joining the front to the back, attaching sleeves, closing side seams. You have three main stitches that work: the mattress stitch for vertical seams (the most common), the backstitch for seams that need to be very strong or where pieces meet at an angle, and the grafting stitch (also called Kitchener stitch) when you want two live edges to join invisibly. Mattress stitch is where most people start because it's forgiving and hides itself inside the fabric.

The choice depends on where the seam is and what you want it to look like. A side seam on a sweater almost always uses mattress stitch. A seam where a sleeve meets the body often uses backstitch because the angle is awkward. Grafting is for joining two edges that are still on needles—the cast-on edge to another cast-on edge, for example, or closing the toe of a sock.

Key Takeaways

  • Mattress stitch is the standard for vertical seams because it runs up the inside of the fabric and is nearly invisible from the outside.
  • You work mattress stitch by threading a tapestry needle under the horizontal bars between stitches on each side, working from the inside out.
  • Backstitch is stronger and works for seams at angles, but it creates a visible line and uses more yarn.
  • Grafting joins two live edges (stitches still on needles) so there is no seam line at all, but it takes practice to keep tension even.
  • Always leave a long tail of yarn when you cast off or bind off—at least 18 inches for most seams—so you have thread to sew with.

Mattress stitch: the standard vertical seam

Mattress stitch works by running your needle under the horizontal bars of yarn that sit between the edge stitch and the next stitch in. These bars are invisible when you're looking at the fabric from the front, but they're there on the inside. You thread under them from one side, then the other, working your way up the seam. The result is a seam that sits inside the fabric and barely shows from the outside.

Start by laying your two pieces flat, right sides facing up, with the edges you're seaming next to each other. Thread a tapestry needle (a blunt needle with a large eye) with the yarn tail from one piece. Insert the needle under the horizontal bar between the first and second stitch on one side, going from right to left. Pull the yarn through, leaving a small loop. Go across to the other piece and insert the needle under the bar in the same position, going from left to right. Pull snug. Go back to the first piece and insert under the next bar up, then across to the second piece and under the next bar up. Keep a steady rhythm: one bar on the left, one bar on the right, moving up the seam.

Every few inches, pull the yarn snug so the seam closes. Don't pull so hard that you pucker the fabric, but pull enough that the two edges come together and the seam disappears into the fold. The tension should match the rest of your knitting. When you reach the top, weave the tail through a few stitches on the inside to anchor it, then trim.

Backstitch for seams that need strength

Backstitch is the traditional sewing stitch and it's much stronger than mattress stitch, but it creates a visible line on the inside of the fabric and uses more yarn. Use it when you're joining pieces at an angle (like a sleeve cap to an armhole), when you need a seam that won't come apart under stress, or when you're joining ribbing to stockinette and mattress stitch won't work cleanly.

Lay your pieces with right sides together (the sides you want to show facing each other). Thread your needle with the yarn tail. Starting at one end, insert the needle through both layers about a quarter-inch from the edge. Bring it back out a quarter-inch further along. Insert it back into the hole you just came out of, then out another quarter-inch ahead. You're always going backward into the last hole, then forward to a new one. Keep your stitches small and even. When you're done, turn the pieces right-side out and the seam will be hidden inside.

Backstitch is slower than mattress stitch and uses more yarn, so reserve it for places where you really need the strength or where the geometry of the pieces makes mattress stitch impossible.

Grafting: joining live edges invisibly

Grafting (Kitchener stitch) joins two edges that still have live stitches on needles, so there is no seam line at all—the fabric appears to continue unbroken. It's the standard way to close the toe of a sock or join a cast-on edge to another cast-on edge. It looks magical when it works, but it requires you to keep both sets of stitches on needles and work slowly.

Hold the two needles parallel, with the working yarn attached to the back needle. Thread a tapestry needle with the working yarn. Insert the tapestry needle through the first stitch on the front needle from right to left (as if to purl), then through the first stitch on the back needle from left to right (as if to knit). Pull the yarn through gently. Now insert through the first front stitch again from left to right (as if to knit) and slip it off the needle. Insert through the next front stitch from right to left. Go to the back needle, insert through the first stitch from right to left (as if to purl) and slip it off. Insert through the next back stitch from left to right. Repeat this pattern all the way across.

The rhythm is: front purl, back knit, front knit (drop), front purl; back purl (drop), back knit. It's straightforward to lose track, so many people count out loud or work very slowly the first time. Keep the tension loose enough that you can still move the stitches on the needles, but snug enough that the grafted stitches match the rest of the fabric.

Preparing your pieces for seaming

Before you seam, make sure you have enough yarn. When you bind off a piece, leave a tail that's at least 18 inches long—longer if you're seaming a large piece or using thin yarn. If you didn't leave a long enough tail, you can thread a new length of yarn through a tapestry needle and use that instead, but it's easier if you plan ahead.

Block your pieces if the pattern calls for it. Blocking means wetting the knitting, pinning it to shape, and letting it dry. This evens out the stitches and makes seaming much cleaner. You don't have to block everything—a straightforward stockinette sweater often doesn't need it—but if your pieces are wavy or uneven, blocking helps.

Lay out all your pieces and make sure you know which edges go together. Mark them with stitch markers or safety pins if it helps. Seaming is much easier when you're not second-guessing which side goes where.

Common mistakes and how to avoid them

The most common mistake is pulling the seam too tight. This puckers the fabric and makes the seam visible. Pull just enough to close the gap between the two pieces, then stop. If you're not sure, err on the side of loose—you can always tighten it later by pulling the tail.

The second mistake is using the wrong stitch for the job. Mattress stitch only works cleanly on vertical seams where both edges are the same type of stitch (both stockinette, both garter, etc.). If you're joining ribbing to stockinette, or if the seam is at an angle, backstitch is usually better. Grafting only works when both edges have live stitches.

The third mistake is not leaving a long enough tail. You can't seam without yarn, and it's frustrating to run out halfway up a seam. Cut a new length and weave it in if you have to, but it's easier to plan ahead.

Finally, don't seam too close to the edge. Leave at least a quarter-inch of fabric between your seam and the edge of the piece. This gives you room to work and keeps the seam from unraveling if a stitch comes loose.

Weaving in ends and finishing

Once your seam is closed, you still have yarn tails to deal with. Thread the tail through a tapestry needle and weave it through the stitches on the inside of the fabric for about 2 inches, going in and out of the loops. This anchors the yarn so it won't come loose. Then weave back in the opposite direction for another inch or so, following a different path through the stitches. This double-weaving keeps the end from popping out. Trim the tail close to the fabric.

If you used a separate length of yarn for seaming (because you didn't have a long enough tail), weave both ends in the same way. Do this for every seam and every tail before you consider the piece finished.

Frequently Asked Questions

Can I use the same yarn color for seaming if it doesn't match perfectly?

Yes, as long as it's close. Mattress stitch hides the yarn inside the fold, so slight color differences won't show. If the yarn is very different, use backstitch instead and hide the seam on the inside of the garment. For grafting, the yarn will show a bit, so match it as closely as you can.

What if I run out of yarn before the seam is finished?

Weave in the end you have, then thread a new length of yarn through the needle and continue. Overlap the old and new yarn by an inch or so, weaving both ends through the same stitches. This keeps the seam from splitting at the join.

How do I fix a seam that's too tight and puckered?

You can try gently pulling the seam from the inside to loosen it, but if it's really puckered, you may need to pick it out and redo it. Use a seam ripper or small scissors to cut the yarn every few stitches, then pull it out and start over. It's tedious, but the result will look better.

Do I have to seam my knitting, or can I knit it in one piece?

You can knit in one piece if you use circular needles or magic loop, but most patterns are written for flat pieces that you seam together. Seaming gives you more control over fit and makes it easier to fix mistakes before you've knitted the whole garment.

What's the difference between a tapestry needle and a regular sewing needle?

A tapestry needle has a blunt tip and a large eye, so it slides between the yarn fibers without splitting them. A regular sewing needle is sharp and has a small eye, which can split your yarn and make the seam look rough. Always use a tapestry needle for knitting seams.