The three main ways to join knitted fabric
You join two finished knitted pieces by sewing them together with yarn and a needle, and the method you choose depends on which edges you are joining and how hidden you want the seam to be. The three most common techniques are the mattress stitch (which runs up the side edges and leaves almost no visible seam), the backstitch (which is faster but creates a thicker, more visible line), and the grafting stitch (which joins two live stitches edge-to-edge so the seam looks like a row of knitting itself). Most projects use mattress stitch for joining sleeves to a body or two body panels together, backstitch for seams that will be hidden inside a garment, and grafting for joining the top of a hat or closing a gap where you want the seam to disappear completely.
Before you join anything, cut a length of yarn about three times the length of the seam you are closing. Thread it onto a yarn needle — a blunt needle with a large eye, not a sharp sewing needle. If your yarn is thin, you can use the same yarn you knitted with; if it is very thick or splitty, use a thinner yarn in a matching color instead.
Key Takeaways
- Mattress stitch is the standard choice for joining side seams because the needle runs between the columns of stitches and the seam stays hidden inside the fold.
- You work mattress stitch from the right side of the fabric, picking up one bar of yarn between stitches on each piece, then sliding the needle up inside the fold before moving to the next row.
- Backstitch creates a visible line but works on any edge and is the fastest method if the seam will be hidden or you do not mind seeing it.
- Grafting joins two rows of live stitches so seamlessly that the join looks like a single row of knitting, but it only works when both edges are still on the needle.
Mattress stitch: the invisible seam for side edges
Mattress stitch works only on vertical edges — the sides of a piece where you cast on and bound off at the top and bottom. Lay your two pieces flat on a table with the right side facing up and the edges you want to join touching but not overlapping. Thread your yarn needle and bring it up through the fabric about half an inch from the bottom of the left piece, coming out between the first and second column of stitches (the columns run vertically up the fabric). This is your starting point.
Now bring the needle across to the right piece and insert it at the same height, between the first and second column on that side. Slide the needle up inside the fold of fabric for about half an inch (roughly one row of knitting), then bring it back out to the surface. Go back to the left piece, insert the needle at the same height where you came out before, and slide it up inside the fold for the same distance. Bring it back out. Repeat this back-and-forth motion all the way up the seam. Every few rows, gently pull the yarn tight — not so tight that you pucker the fabric, but tight enough that the two edges draw together and the seam disappears into the fold.
When you reach the top, weave the yarn tail back down through a few stitches on one side to find it, then trim. The seam should be nearly invisible from the right side because the needle never pierces the outer loops of the stitches — it runs in the space between the columns.
Backstitch: the fast seam for hidden or visible joins
Backstitch works on any edge — vertical sides, horizontal cast-on or bind-off edges, or even diagonal edges. Place your two pieces with right sides together (so the wrong sides face out) and line up the edges you want to join. Thread your yarn needle and bring it up through both layers about a quarter inch from the corner, then back down a quarter inch further along the seam. Bring it back up a quarter inch ahead of that, then back down into the hole you just came out of. This creates a line of overlapping stitches that locks the two pieces together.
Backstitch creates a seam that is about a quarter inch thick and visible as a line of yarn running along the join. This is fine if the seam will be hidden inside the garment (like the underarm seam of a sweater), but if you want an invisible seam on a visible edge, use mattress stitch instead. Backstitch is faster than mattress stitch because you do not have to slide the needle inside the fold — you just stab straight through both layers — so it is a good choice when speed matters more than appearance.
Grafting: joining live stitches so the seam vanishes
Grafting works only when both pieces have live stitches still on the needle — you have not bound them off yet. This technique is most common when closing the top of a hat or joining the two halves of a seamless sweater that you knitted in pieces. Hold the two needles parallel to each other with the right sides of the fabric facing you. Thread your yarn needle with a length of yarn about three times the length of the stitches you are grafting.
Bring the yarn needle up through the first stitch on the front needle from right to left (so the needle comes out on the left side of the stitch), then down through the first stitch on the back needle from left to right. Bring it back up through the second stitch on the back needle from right to left, then down through the second stitch on the front needle from left to right. Continue this pattern — always going up through the back needle and down through the front needle, moving one stitch forward each time — all the way across. When you finish, drop the stitches off both needles. The join will look like a single row of knitting running between the two pieces.
Grafting takes practice to get the tension right — too loose and the join will gap, too tight and it will pucker. Work slowly and check your tension every few stitches by gently pulling the yarn. If you make a mistake, it is easier to catch than with backstitch because you can unravel just the grafted row and start again without disturbing the knitted fabric.
Preparing your pieces before joining
Before you join anything, make sure both pieces are the same size and shape where they will meet. If one piece is slightly larger, you can ease the extra fabric in as you sew by taking slightly smaller stitches on the larger piece. If one piece is significantly larger, you may need to block both pieces to the same dimensions — wet them, pin them to shape on a blocking board, and let them dry. This is especially important if your yarn is wool or another natural fiber that responds well to blocking.
Check that you are joining the correct edges. If you are joining two body panels, you should be joining the side edges (the vertical edges). If you are joining a sleeve to a body, you are joining the bind-off edge of the sleeve cap to the armhole edge of the body, which usually requires a combination of backstitch and mattress stitch because one edge is horizontal and one is vertical. Lay the pieces out and trace the seam with your finger before you start sewing so you know exactly where the needle should go.
Fixing mistakes and adjusting tension
If you pull the yarn too tight as you join, the seam will pucker and the fabric around it will bunch up. If you pull too loose, the seam will gap and the two pieces will not hold together. The right tension is snug enough that the edges draw together but loose enough that you can still see the individual stitches on either side of the seam. As a rule, pull the yarn tight after every two or three rows, not after every single stitch.
If you make a mistake in the middle of a seam, you can usually unravel just a few stitches, pull the yarn back out, and start again from where the mistake began. With mattress stitch, this is straightforward because the stitches sit in the fold and do not affect the knitted fabric itself. With backstitch, you may need to carefully cut the yarn and pull it out stitch by stitch. With grafting, you can unravel the entire grafted row and start over without damaging the knitted pieces.
Frequently Asked Questions
Can I join pieces if one edge is cast on and the other is bound off?
Yes, but the seam will be slightly thicker because the cast-on and bind-off edges are denser than the side edges. Use backstitch for this join because it works on any edge. If you want a less visible seam, you can use mattress stitch if the edges run vertically, but the seam will be slightly bulkier than a side-to-side join.
What yarn should I use to join the pieces?
Use the same yarn you knitted with if it is smooth and not too thick. If your yarn is very fuzzy, splitty, or thick, use a thinner yarn in a matching color instead — it will be easier to thread through the needle and will not create a thick, visible seam. Test the yarn on a scrap piece first to make sure the color matches when the yarn is wet.
How do I join pieces if the edges are not the same length?
Ease the extra fabric in by taking slightly smaller stitches on the longer piece as you sew. This spreads the extra length across the entire seam instead of bunching it up in one spot. If one piece is much longer than the other, block both pieces to the same dimensions before you join them.
Do I need to block the pieces before joining them?
Blocking is not required, but it helps the pieces hold their shape and makes the edges easier to join. If your yarn is wool or another natural fiber, blocking will also even out any uneven stitches and make the seam look neater. If your yarn is acrylic or synthetic, blocking may not make much difference.
Can I use a sewing machine to join knitted pieces?
A sewing machine can work for backstitch seams, but it is not recommended for mattress stitch or grafting because the machine cannot navigate the folds and live stitches the way a hand needle can. Hand sewing gives you much more control over tension and seam placement, so it is the standard method for joining knitted pieces.