The three main stitches for joining knit pieces

The most common way to join knit pieces is the mattress stitch, which creates an invisible seam by running thread up the sides of your stitches rather than through them. You work from the right side of the fabric, which means you can see exactly how the seam looks as you go. For most sweaters, scarves, and garments, mattress stitch is the first choice because the finished seam is flat and nearly invisible.

The backstitch is faster but leaves a visible ridge on the inside of your garment. It works well for seams you will not see — like the underside of an armhole or the back of a collar — and for joining pieces where a small amount of bulk does not matter. You sew through both layers at once, one stitch at a time, working from the inside of the garment.

The whip stitch (also called ladder stitch) is similar to mattress stitch but works better for closing gaps in ribbing or for seaming pieces that have very loose edges. It creates a flexible seam that moves with the knit, which matters if your finished piece needs to stretch.

Key Takeaways

  • Mattress stitch is invisible from the outside and works from the right side of your fabric, making it the standard choice for visible seams on sweaters and fitted garments.
  • You pick up one bar of yarn between stitches on each side, then pull the thread tight every few rows to close the gap.
  • Backstitch is faster but leaves a visible seam on the inside, so use it only where the seam will not show or where bulk is acceptable.
  • Thread tension matters more than speed — pulling too hard distorts the knit, while pulling too loose leaves gaps that will not close.
  • A blunt yarn needle (not a sharp sewing needle) prevents you from splitting the yarn and creating weak spots in the seam.

How to work mattress stitch step by step

Start by laying your two pieces flat on a table with the right sides facing up and the edges you want to join running parallel to each other. Thread a blunt yarn needle with a length of yarn about an arm's length long — if it is too long, it will tangle and fray as you work. Tie a small knot at the end or weave the needle through a few stitches on one piece to anchor it.

Insert the needle under the first horizontal bar (the strand of yarn that runs between two vertical columns of stitches) on the left piece, two rows up from where you started. Pull the yarn through, then cross to the right piece and insert the needle under the corresponding bar. Come back out one bar down. Return to the left piece and insert under the bar directly above where you came out, then come out one bar down. Repeat this pattern — always going under one bar and coming out one bar down — working your way up the seam.

Every few rows (usually after four to six stitches), pull the working yarn snug. The two edges will draw together and close the gap. Do not pull so hard that the fabric bunches or distorts — the seam should close smoothly without puckering. If you pull too loose, the gap will remain open and the seam will be weak.

When you reach the top, weave the needle through a few stitches on the inside of the fabric to find it, then trim the yarn. The finished seam should be nearly invisible from the right side and create a smooth, flat join.

Backstitch for seams that do not need to be invisible

Backstitch works best for seams on the inside of a garment or for joining pieces where a small ridge of bulk is acceptable. Lay your pieces with wrong sides together (the sides you do not want to see facing each other). Thread your needle and tie a knot.

Starting at one end, insert the needle through both layers about one-quarter inch from the edge. Bring it back out one-quarter inch ahead of where you started. Insert the needle back into the hole you just came out of, then bring it out one-quarter inch ahead again. Each new stitch goes back into the hole of the previous stitch, creating a solid line of thread. Work your way along the seam, keeping your stitches even and your tension consistent.

Backstitch creates a seam that is sturdy and does not stretch, which makes it useful for joining pieces that need to hold their shape. The trade-off is that it leaves a visible line of thread on the inside of the garment, and the seam itself takes up a small amount of space that mattress stitch does not.

Whip stitch for flexible seams and ribbing

Whip stitch is a good choice when you are joining ribbed edges or when you need the seam to stretch along with the knit. Lay your pieces with right sides facing up, edges parallel, just as you would for mattress stitch. Thread your needle and anchor it on one piece.

Insert the needle under the first bar on the left piece, then bring it across and insert under the corresponding bar on the right piece. Come back out, then return to the left piece and insert under the next bar up. The needle travels in a shallow diagonal line across the gap between the two pieces, picking up one bar on each side before moving to the next row. Pull the thread snug every few stitches to close the gap.

Whip stitch creates a seam that is slightly more visible than mattress stitch but more flexible. It works especially well for joining the edges of ribbing because the seam moves with the stretch of the rib rather than fighting against it.

Choosing the right yarn and needle for seaming

Always use a blunt yarn needle (also called a tapestry needle) rather than a sharp sewing needle. A sharp needle will split the yarn as you push it through, which weakens the seam and makes it harder to work. Blunt needles have a rounded tip that pushes the yarn aside instead of piercing it.

Use the same yarn you knitted with if you have leftover yarn from your project. If you do not, match the weight and fiber as closely as you can. Yarn that is too thin will not hold the seam securely; yarn that is too thick will create visible bumps. If your yarn is very fuzzy or delicate, you can use a single strand of a similar-weight yarn in a matching color instead.

Avoid using thread from a sewing kit. Thread is too thin and will not hold a knit seam the way yarn does. It will also break more easily as the knit stretches and moves.

Common mistakes and how to avoid them

The most common mistake is pulling the seaming yarn too tight. This distorts the shape of the knit and creates puckers or bunches along the seam. Pull just snug enough that the gap closes — the seam should look smooth and flat, not pinched. If you notice puckering, loosen the last few stitches and try again with less tension.

Another mistake is using a sharp needle or thread instead of a blunt yarn needle. This splits the yarn and creates weak spots that will unravel or break under stress. A blunt needle is worth buying if you do not have one — it makes the work faster and the seam stronger.

Seaming too tightly also happens when you pick up the wrong bar of yarn. On stockinette stitch (the most common knit pattern), each stitch has a horizontal bar running between the columns. Make sure you are picking up the bar between the edge stitch and the next stitch in, not the bar at the very edge of the fabric. The very edge bar is too loose and will not hold the seam.

Finally, do not rush. Seaming is slower than knitting, but taking your time means you can see what you are doing and catch mistakes before they become problems. If a seam does not look right, it is easier to fix it while you are still working than to try to redo it later.

When to block your seams

Blocking is the process of wetting your finished garment and pinning it to shape while it dries. For seamed pieces, blocking helps even out the seams and makes them less visible. You do not have to block every seam, but blocking is especially helpful if your seams look uneven or if you want the finished garment to have a polished look.

To block a seam, wet the finished garment (or just the seamed area) with cool water, gently squeeze out the excess, and pin it to a blocking board or towel in the shape you want it to be. Let it dry completely — this usually takes 24 hours or more depending on the yarn weight and fiber. Once dry, the seams will be flatter and more even, and the yarn will have relaxed into its final shape.

Frequently Asked Questions

What if my seam has gaps or holes in it?

Gaps usually mean you pulled the seaming yarn too loose or you missed picking up a bar on one side. You can go back and tighten those stitches by pulling the working yarn snug. If the gap is small, you can also use a crochet hook to pull the working yarn through the gap from the inside, which will close it without having to redo the whole seam.

Can I use mattress stitch on ribbed edges?

Mattress stitch works on ribbing, but whip stitch is usually better because it creates a more flexible seam. If you use mattress stitch on ribbing, pick up the bars between the knit and purl stitches rather than the bars within the ribbed columns, so the seam stretches along with the rib.

How do I hide the knot at the start of my seam?

Instead of tying a knot, weave the needle through a few stitches on the inside of one piece before you start seaming. This anchors the yarn without creating a knot that might show or come loose. Do the same at the end of the seam to find it.

What if the two pieces I am joining are different sizes?

Measure both pieces carefully before you start. If one is slightly larger, you can ease the extra fabric in by picking up slightly fewer bars on the larger piece — this distributes the extra length evenly along the seam instead of bunching it in one spot. If the pieces are very different sizes, you may need to adjust your pattern or reknit one piece.

Do I need to seam if I am using a circular needle?

Circular needles let you knit in the round, which means you can avoid seaming altogether on pieces like sweater bodies and hats. However, you will still need to seam sleeves to the body, and you may need to seam the underarm if you knit the sleeves separately. Check your pattern to see which seams are required.