What Sewing a Seam in Knitting Means
Sewing a seam in knitting joins two finished pieces—a sleeve to a body, two halves of a sweater front, or the sides of a hat—using a needle and yarn instead of the knitting needles themselves. The most common method is the mattress stitch, which runs a thread up the inside of the edge stitches on both pieces, pulling them together from the inside so the seam is nearly invisible from the outside. Other methods include the backstitch (visible but strong) and the grafting stitch (which makes the seam look like a continuous row of knitting).
The choice of method depends on where the seam will be—a side seam that will be hidden inside a garment is a good place for mattress stitch, while a seam on a visible edge might call for grafting. You'll need a tapestry needle (blunt, with a large eye), yarn that matches your knitting, and a flat surface to work on.
Key Takeaways
- The mattress stitch is the most common seaming method because it creates an invisible seam by running thread through the inside edges of both pieces.
- You thread a tapestry needle with a length of yarn (usually 18 to 24 inches) and work from the inside of the pieces, never piercing the actual stitches themselves.
- The mattress stitch works best on stockinette stitch fabric and requires you to count rows carefully so both sides stay aligned.
- A backstitch seam is visible but faster and works well on ribbed edges or when you want a decorative seam on the outside of a garment.
- Grafting creates a seam that looks like a continuous knitted row and works only when you have live stitches (stitches still on the needle) on both pieces.
Preparing Your Pieces and Yarn
Before you start sewing, lay both pieces flat on a table with the right sides (the outside of the finished knitting) facing up. Line them up so the edges you're joining are parallel and close together—you don't need them touching, but they should be within an inch of each other. If the pieces are different lengths, pin them at the top and bottom so they don't shift while you work.
Cut a length of yarn about 18 to 24 inches long—long enough to sew a seam without running out, but short enough that it won't tangle. Thread it through a tapestry needle. If you're using the same yarn you knitted with, pull a strand from the ball or from the end of a finished piece. If the yarn is too thick or fragile, you can use a thinner yarn in a matching color, though matching the original yarn is usually best.
The Mattress Stitch: Step by Step
Start at the bottom of your seam. Insert the needle under the first horizontal bar (the strand of yarn that runs between two vertical stitches) on the right piece, going from right to left. Pull the yarn through, leaving a short tail of about 2 inches—you'll weave this in later. Now insert the needle under the corresponding bar on the left piece, again going from right to left, and pull through.
Go back to the right piece and insert the needle under the next bar up (the one above where you just came out). Pull through. Then go to the left piece and insert under the next bar up there. Keep alternating: right piece, left piece, right piece, left piece, moving up one bar each time. After every few stitches, gently pull the working yarn to draw the two pieces together. The seam will start to close as you pull, and the edges will meet in the middle.
Continue until you reach the top of the seam. Fasten off by running the needle through a few stitches on the inside of one piece, then weave the yarn tail back down through the seam itself so it disappears. Trim any excess yarn.
The Backstitch Seam for Visible Edges
The backstitch creates a visible seam on the outside of your knitting, so it works well when you want a decorative line or when you're joining ribbed edges where the mattress stitch doesn't work as cleanly. Place the two pieces with right sides together (the outside facing inward, toward each other) and align the edges.
Thread your tapestry needle and insert it through both layers about half a stitch in from the edge. Bring it back out half a stitch further along the seam. Now insert the needle back into the hole you just came out of, going through both layers again, and bring it out another half stitch ahead. This creates a line of overlapping stitches that looks like a backstitch in sewing. Pull gently as you go so the seam stays even, but not so hard that you pucker the fabric.
Grafting: Seaming Live Stitches Together
Grafting works only when both pieces have live stitches—stitches still on the needle or on a stitch holder—rather than bound-off edges. It creates a seam that looks like a continuous row of knitting, with no visible line between the two pieces. This is often used to close the top of a hat or to join the front and back of a sweater at the shoulders.
Hold the two needles parallel, with the right sides of the knitting facing you. Thread a tapestry needle with a length of 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). Go back to the front needle and insert through the same stitch from left to right, then through the next stitch from right to left. Repeat this pattern—alternating between front and back needles, and alternating the direction you insert—until all stitches are grafted. As you work, gently pull the working yarn so the seam closes and the stitches align.
Fixing Common Seaming Mistakes
If your seam is puckered or uneven, you've likely pulled the yarn too tight. Undo the seam back to where it looks even, loosen your tension, and try again. If one side is longer than the other, you may have skipped a row on one piece. Count the rows as you go: after every few stitches, count up from the bottom on both pieces to make sure you're at the same height.
If the seam is visible when it shouldn't be, you may have been piercing the actual stitches instead of running under the bars between them. The mattress stitch should run under the horizontal bars, not through the vertical loops. If you've already sewn it and it's too visible, you can undo it and try again—the yarn will have a slight kink, but it will still work.
When to Call in Help or Start Over
Seaming is a skill that improves with practice, and your first seam will likely be rougher than your fifth. If you've sewn a seam and it's pulling the pieces out of shape, or if the two sides are noticeably different lengths, it's worth undoing and trying again rather than living with a garment that doesn't fit or look right. Yarn is cheap; wearing an uncomfortable sweater is not.
If you're working with a delicate or slippery yarn (like silk or very thin merino), you may want to use a slightly thicker yarn for seaming so you have better control. Some knitters use thread designed for sewing instead, though this is less common and works best on very fine knitting.
Frequently Asked Questions
Can I use the same yarn I knitted with for seaming?
Yes, and it's usually the best choice because the color and texture will match perfectly. Pull a strand from the ball or from the end of a finished piece. If the yarn is very thick or fragile, a thinner yarn in the same color will work, but matching the original is ideal.
How tight should I pull the seaming yarn?
Pull gently after every few stitches so the two pieces draw together and the seam closes, but not so hard that you pucker the fabric or distort the stitches. If your seam looks wavy or bunched, you're pulling too tight. Loosen it and try again.
What if I don't have a tapestry needle?
A tapestry needle is designed to be blunt so it won't split the yarn, but in a pinch you can use a regular sewing needle with a large eye. Be careful not to pierce the yarn strands themselves—you want to run under them, not through them.
Can I seam ribbed edges with the mattress stitch?
The mattress stitch works better on stockinette stitch than on ribbing. For ribbed edges, a backstitch seam is usually cleaner and faster. If you want to use mattress stitch on ribbing, work under the bars of the knit stitches only, skipping the purl stitches.
What do I do with the yarn tail after I finish seaming?
Weave the tail back through the seam itself using the tapestry needle, running it up and down through the stitches for an inch or so. This anchors it so it won't come loose. Trim any excess yarn close to the fabric.