The two main ways to join knitted pieces
You have two practical routes: mattress stitch, which creates an invisible seam by working through the edge loops, and backstitch, which is faster but leaves a visible line. Mattress stitch is the standard for garments because the seam disappears into the fabric. Backstitch works well for seams you want to be sturdy—like the sides of a bag or a blanket where the seam won't show on the outside.
Both methods require a yarn needle (also called a tapestry needle), which has a blunt tip and an eye large enough for yarn. You'll use the same yarn you knitted with, or a matching color if you're joining different shades. The key difference is where your needle goes: mattress stitch stays inside the loops at the edge, while backstitch pierces through both layers of fabric.
Key Takeaways
- Mattress stitch creates an invisible seam by working through the loops along the edge of your knitting, making it the best choice for visible seams on sweaters and fitted pieces.
- Backstitch is faster and creates a sturdy seam, but it leaves a visible line and works best for seams that won't show or for pieces that need extra strength.
- You need a yarn needle with a blunt tip, matching yarn, and pieces with at least one row of stockinette stitch at the edge to work with either method.
- Mattress stitch requires you to count bars between rows and pick up the same number of bars on each side so the seam stays even and doesn't pucker.
- Blocking your pieces before seaming makes them lie flat and easier to join, and it helps the finished seam sit smoothly against the fabric.
How to work mattress stitch step by step
Start by laying your two pieces flat with the right sides facing up—you're looking at the outside of the fabric. Thread your yarn needle with a length of yarn about 18 to 24 inches. Tie a knot at the end and bring the needle up through the bottom corner of the left piece, one stitch in from the edge. This anchors your thread.
Now look at the right piece. Between the first and second columns of stitches, you'll see a horizontal bar running under the stitch. Insert your needle under that bar from right to left, then go back to the left piece and insert the needle under the bar in the same row. Pull gently—not tight—and move up one row. Repeat this back-and-forth motion, always picking up the bar between the first and second columns. After every few rows, pull the yarn snug so the seam closes, but not so hard that it bunches the fabric.
The seam will look like a ladder while you're working. When you pull the yarn tight at the end, the ladder closes and the seam vanishes into the fabric. This is why mattress stitch takes practice—you're trusting that the seam will disappear once you tighten it. If your seam is puckering, you picked up a different number of bars on each side, so go back and adjust.
How to work backstitch for a strong, visible seam
Lay your pieces with right sides together (the outside of the fabric facing each other). Thread your needle and bring it up through both layers about a quarter-inch from the edge. This quarter-inch margin is your seam allowance and keeps the seam from sitting right at the edge where it can unravel.
Push the needle back through both layers one stitch to the left, then bring it forward two stitches to the right and push back through again. You're always going back one stitch and forward two, which overlaps your stitches and creates a strong line. Keep the stitches small and even—about the same size as your knitted stitches. When you're done, the seam will be visible on the inside of the piece, and the outside will look like a clean line.
Backstitch works especially well for joining ribbed edges or for seams that will take stress, like the underarm of a sweater or the bottom of a bag. It's also faster than mattress stitch because you don't have to count bars or worry about the seam closing properly.
Preparing your pieces so the seam sits flat
Before you seam, block your pieces by pinning them to shape on a blocking board or towel, misting them lightly with water, and letting them dry. Blocking relaxes the yarn and makes the edges even, which means your seam won't have to do extra work to pull the pieces into alignment. Pieces that aren't blocked often have wavy edges, and seaming them can make the waviness worse.
Check that your edges are straight and that you have at least one full row of stockinette stitch (the smooth V-pattern) at the seam line. If your piece ends in ribbing or garter stitch, mattress stitch becomes harder because there are no clear bars to pick up. In that case, backstitch is your better option.
Fixing seams that pucker or twist
If your seam is puckering, the most common cause is pulling the yarn too tight. Mattress stitch should close gently—imagine the seam is closing on its own and you're just guiding it. If you're pulling hard enough to see the fabric bunch, loosen up. You can also try picking up fewer rows at a time before you pull tight, which gives you more control.
If your seam is twisting or spiraling, you may have picked up a different number of bars on each side. Undo a few inches, count the bars on both pieces to make sure they match, and start again. This is tedious but much faster than ripping out the whole seam and starting over.
If the seam is visible when it shouldn't be, you may have worked too far in from the edge. Mattress stitch works best when you're picking up the outermost bar on each side. If you went one bar too far in, the seam will sit inside the fabric instead of at the edge.
Joining pieces that don't line up perfectly
Sometimes one piece is slightly longer than the other, or the rows don't match up exactly. If the difference is small—one or two rows—you can hide it by picking up bars unevenly. On the longer piece, skip a bar every few rows. This stretches the seam slightly and brings the pieces into alignment without creating a visible lump.
If the difference is larger, you may need to rip back a few rows on the longer piece and reknit it to match, or accept that the seam will have a small step. A step is less noticeable than a pucker, and it's often invisible once the garment is worn and the fabric relaxes.
Seaming cast-on and bind-off edges
Joining the top and bottom edges of a piece (like closing the sides of a sweater) uses the same mattress stitch, but you're picking up the bumps along the cast-on or bind-off edge instead of the bars between stitches. These bumps are less obvious than bars, so work slowly and use good lighting. If you can't see the bumps clearly, try using a smaller needle or working under a lamp.
Backstitch also works for these edges and may be easier if you're having trouble seeing the bumps. The seam will be visible on the inside, but it will be strong and you won't have to hunt for the right place to insert the needle.
Frequently Asked Questions
Can I use the same yarn I knitted with, or do I need something special?
Use the same yarn if you have it left over. If you don't, match the weight and fiber as closely as you can. A yarn that's too thick will make the seam bulky; one that's too thin will be weak. If the color doesn't match exactly, a slightly darker shade is less noticeable than a lighter one.
What if I seamed two pieces and they don't line up?
If the misalignment is small, you can often hide it by blocking the finished piece. Pin it to shape and let it dry, and the fabric will relax and settle. If the misalignment is large, you may need to rip out the seam and try again, or accept it as part of the handmade character of the piece.
Do I have to block before seaming?
Blocking makes seaming easier and the finished seam look better, but it's not required. If you're in a hurry, you can seam unblocked pieces. Just expect the edges to be less even and the seam to need more adjustment as you work.
Is mattress stitch always better than backstitch?
Mattress stitch is invisible, so it's the standard for seams you'll see on the outside of a garment. Backstitch is faster and stronger, so it's better for seams that won't show or for pieces that need to withstand stress. Choose based on where the seam will be and how much time you have.
How do I know if my seam is tight enough?
The seam should be snug enough that the two pieces don't shift when you gently pull them, but loose enough that the fabric doesn't bunch or pucker. If you can see the yarn between the stitches, it's too loose. If the fabric is bunching, it's too tight. This takes practice, so don't worry if your first seams aren't perfect.