What seaming is and why you need it
Seaming is the method you use to join two finished pieces of knitting—a sleeve to a body, a front panel to a back, or two halves of a hat. Instead of binding off and leaving raw edges, you use a needle and yarn to close the gap between pieces so the seam is nearly invisible and blends into the fabric itself.
Most knitting patterns that make a garment require seaming. You knit flat pieces separately, then seam them together. The seam itself becomes part of the finished garment, so learning to seam neatly makes the difference between a project that looks handmade and one that looks professionally finished.
Key Takeaways
- The mattress stitch is the most common seaming method because it creates an invisible seam that runs between the edge stitches, not through them.
- You seam with the right sides of the fabric facing you, using a blunt needle and the same yarn weight as your project.
- The basic motion is to pick up one bar of yarn between stitches on one side, then one bar on the other side, then pull gently to close the gap.
- Seaming works best when you leave at least one stitch unbound on each edge—this gives you the bars of yarn to work into.
Preparing your pieces for seaming
Before you seam, lay both pieces flat on a table with the right sides facing up. Line up the edges you plan to join so they sit next to each other without stretching or bunching. If the edges look wavy or uneven, you may have tension issues in your edge stitches, but seaming can still work—just be gentle.
Check that you have left selvage stitches (edge stitches) on both pieces. A selvage stitch is the first or last stitch of each row, and it sits slightly loose compared to the stitches next to it. This loose stitch is where the bars of yarn live that you will use for seaming. If you bound off all the way to the edge, you have no bars to work with, and the seam will be bulky and visible.
Thread a blunt tapestry needle with a length of yarn about 18 to 24 inches long. Use the same yarn you knitted with, or yarn that matches the weight and fiber. If your project used a variegated yarn, a solid color that matches one of the colors in the variegation usually blends in better than the variegated yarn itself.
The mattress stitch: the standard seaming method
The mattress stitch is the seam you will use most often. It runs vertically between the edge stitches of two pieces, and when done correctly, it is nearly invisible from the front of the garment.
Start by inserting your needle under the first horizontal bar of yarn on one piece, between the first and second stitches from the edge. Pull the yarn through, leaving a short tail (about 2 inches). Then insert the needle under the corresponding bar on the other piece, in the same position relative to the edge. Pull gently. Go back to the first piece and pick up the next bar up, then the next bar on the second piece. Continue this pattern—one bar on the first piece, one bar on the second piece—moving upward along the seam.
Every few stitches, pull the working yarn gently to draw the two pieces together. The seam should close smoothly without puckering. If you pull too hard, the seam will pucker and the fabric will bunch. If you do not pull at all, the seam will be loose and floppy. The right tension feels like a gentle draw that closes the gap without distorting the fabric.
Seaming horizontal edges (like a shoulder seam)
When you seam a horizontal edge—such as joining a shoulder seam where both pieces end in a bound-off edge—the motion changes slightly. Instead of picking up bars between stitches, you pick up the bumps (the horizontal loops) that sit just below the bound-off edge on each piece.
Insert your needle under one bump on the first piece, then under the corresponding bump on the second piece. Work across the seam in this way, pulling gently every few stitches to close the gap. The seam will be thicker than a vertical mattress stitch because you are working with bound-off edges, but it will still be relatively neat if you keep your tension even.
Seaming curved or angled edges
Armholes and necklines are curved, and seaming a curve requires you to ease the fabric slightly. As you seam, you may find that one piece is slightly longer than the other. Distribute the extra length evenly across the seam by occasionally picking up two bars on the longer piece and one bar on the shorter piece. This eases the extra fabric in gradually rather than bunching it all in one spot.
Work slowly on curved seams and check your progress frequently. Lay the pieces flat and look at the seam from the front to make sure it is not puckering. If it is, you are pulling too hard or easing unevenly. Undo a few stitches and try again.
Finishing the seam and weaving in the tail
When you reach the end of the seam, insert your needle through the last stitch on one piece, then back through the working yarn a few times to lock it in place. Trim the working yarn, leaving about a 2-inch tail.
Go back to the beginning of the seam and do the same with the starting tail. Weave both tails into the seam itself using your tapestry needle, running the needle through the bars of yarn inside the seam for about an inch. This hides the tails completely. Trim any excess yarn.
Common seaming mistakes and how to fix them
The most common mistake is seaming through the edge stitches themselves instead of between them. This creates a thick, visible seam and makes the fabric stiff. If you do this, the seam will look like a ridge running down the side of your garment. The fix is to undo the seam and start over, making sure you are picking up the bars of yarn that sit between the edge stitch and the next stitch inward.
Another mistake is pulling the seam too tight. This makes the fabric pucker and can actually shorten the seam, making the pieces no longer fit together properly. If this happens, undo the seam, and this time pull only gently—just enough to close the gap without distorting the fabric.
Uneven tension is also common, especially when you are learning. Some stitches will be tight and some loose, making the seam look bumpy. The fix is to practice on scrap fabric first, focusing on keeping your pull consistent from stitch to stitch.
Frequently Asked Questions
Can I seam with a different yarn than the one I knitted with?
Yes, as long as the weight is similar. A yarn that is too thin will disappear into the seam and not hold it together. A yarn that is too thick will make the seam bulky and visible. A solid color that matches the main color of your project usually works best, especially if your project yarn was variegated or multicolored.
What if I bound off all the way to the edge and have no bars to seam with?
You can still seam, but the seam will be thicker and more visible. Work through the bound-off stitches themselves, picking up the bumps just below the bind-off edge. It is not ideal, but it will hold. For future projects, leave one selvage stitch unbound on each edge to make seaming easier.
How do I seam a ribbed edge?
Ribbed edges have a pattern of knit and purl stitches, so the bars of yarn are less obvious. Work slowly and look closely at where the bars sit. You may need to pick up bars from the knit stitches in the rib rather than trying to seam through the purl bumps. The seam will be slightly more visible than on a stockinette edge, but it will still be neat if you are careful.
What if my two pieces are different lengths?
Ease the longer piece into the shorter one by occasionally picking up two bars on the longer side and one on the shorter side. Distribute the extra length evenly across the seam so no single spot bunches up. If the difference is more than a quarter inch, the pieces may not have been knitted to the same gauge, and you may need to block them to even them out before seaming.
Do I have to use the mattress stitch?
No. The backstitch and the whip stitch are alternatives, though they create more visible seams. The mattress stitch is the standard because it is nearly invisible and creates a strong, flexible seam that moves with the fabric. Once you learn it, you will likely use it for most projects.