The three main ways to join knitted fabric

You join two finished knitted pieces by sewing them together with yarn and a needle. The method you choose depends on where the seam will be (edge to edge, or edge to a flat surface), how visible it needs to be, and how much stretch the seam requires. The three most common techniques are the mattress stitch (invisible seam on stockinette), the backstitch (strong seam that shows), and the whip stitch (for binding off edges to flat surfaces).

Each method uses the same basic tool: a yarn needle (also called a tapestry needle) threaded with yarn that matches your knitted fabric. The needle has a blunt tip so it slides between yarn strands without splitting them. You work from the inside of the garment when possible, so the seam stays hidden.

Before you start any seam, lay both pieces flat on a table and line up the edges you are joining. Pin them in place if the pieces are large or if you are nervous about alignment. Thread your needle with a length of yarn about an arm's length long — longer than that and it tangles; shorter and you run out mid-seam.

Key Takeaways

  • The mattress stitch creates an invisible seam on stockinette fabric by running your needle under the horizontal bars between stitches on each side.
  • The backstitch produces a visible, sturdy seam and works on any stitch pattern, but it uses more yarn and sits on the outside of the garment.
  • The whip stitch joins a live edge (stitches still on the needle) to a bound-off edge or flat surface by catching one stitch on each piece per pass.
  • Matching yarn color and weight to your knitted pieces prevents the seam from standing out or creating weak spots.
  • Blocking your pieces before seaming makes edges even and seaming faster, because the pieces stay flat and aligned.

Mattress stitch for invisible seams on stockinette

The mattress stitch is the standard choice for joining stockinette pieces edge to edge because the seam disappears into the fabric. It works by running your needle under the horizontal bars (called running threads) that sit between the edge stitch and the next stitch inward. These bars are visible on the side of stockinette fabric that faces you when you hold the piece flat.

Start by threading your needle with yarn and tying a small knot at the end. Insert the needle from the back of one piece, coming out between the first and second stitch from the edge. Go across to the other piece and insert the needle between the first and second stitch on that side, going in from the front. Bring the needle out one stitch to the right (or left, depending on your direction). Go back to the first piece and repeat: insert where you came out, come out one stitch over.

Work in a rhythm: one pass on the left piece, one pass on the right piece, moving up the seam. Every few passes (usually every half inch or so), pull the yarn snug but not tight — tight enough that the two pieces draw together, but not so tight that you pucker the fabric. The seam will close as you work, and the edges will meet in the middle. When you reach the top, weave the yarn tail through a few stitches on the inside and trim it.

Backstitch for strong, visible seams

The backstitch is faster than the mattress stitch and works on any stitch pattern, but it creates a visible seam on the outside of your garment. Use it for seams that will be hidden (like the sides of a sweater under the arm) or when you want the seam to be a design feature. The backstitch is also the strongest seam for pieces that will take stress, such as the sides of a bag.

Pin the two pieces together with the right sides facing each other (the sides you want to show on the finished garment facing inward). Thread your needle and tie a knot. Insert the needle through both layers at one end of the seam, coming out about one-quarter inch from the edge. Bring the needle back to where you started and insert it there, then come out one-quarter inch ahead. Go back and insert the needle where you just came out, then come out one-quarter inch ahead again. This back-and-forth motion creates the backstitch.

Keep your stitches small and even — about one-quarter inch apart — so the seam is strong and does not pucker. When you reach the end, weave the yarn tail through a few stitches on the inside and trim it. Turn the garment right-side out and the seam will be visible on the outside.

Whip stitch for joining live edges to bound-off edges

The whip stitch joins a live edge (stitches still on your needle) to a bound-off edge or to a flat surface like a picked-up edge. It is common when attaching a neckband to a sweater or joining the sides of a collar. The whip stitch is nearly invisible because it only catches a single stitch on each piece per pass.

Hold the piece with the live stitches in your left hand and the bound-off piece in your right. Thread your needle with yarn. Insert the needle under one stitch on the live edge (going from right to left under the stitch), then across to the bound-off piece and under one stitch there (going from left to right). Come back to the live edge and repeat. Each pass catches one stitch on each piece.

Pull the yarn snug after every few stitches so the seam draws the pieces together without puckering. Work all the way across the edge, then weave the tail through a few stitches and trim it. The seam will be nearly invisible because it sits inside the fold where the two edges meet.

Preparing pieces and choosing yarn for seaming

Before you seam, block your finished pieces if your pattern calls for it. Blocking wets the fabric, stretches it to the correct dimensions, and lets it dry in shape. Blocked pieces have even edges and sit flat, which makes seaming faster and produces a neater result. If you do not block, at least lay your pieces flat and let them rest for a few hours so they are not curled or wavy.

Use yarn that matches your knitted fabric in both color and weight. If you used a variegated yarn (multicolored), use a solid color that matches the dominant shade, or use the same variegated yarn if you have leftovers. If your yarn is thick and textured, use the same yarn for seaming. If your yarn is thin, you can use a slightly thicker yarn for seaming as long as the color matches — the seam will be stronger and less visible.

Cut a length of yarn about 36 inches long and thread it onto your yarn needle. If you are seaming a long seam, you may need to add yarn partway through by weaving a new length into the seam and continuing. This is normal and does not weaken the seam if you overlap the old and new yarn by at least an inch.

Fixing common seaming mistakes

If your seam is too tight and puckering the fabric, you pulled the yarn too hard. Undo the last few stitches, loosen the tension, and try again. A seam should draw the pieces together but not distort the fabric around it. If your seam is too loose and the pieces are separating, you did not pull hard enough. Undo and tighten the tension on the next attempt.

If your mattress stitch seam is visible instead of invisible, you may be catching the wrong threads. The mattress stitch works only on stockinette fabric and only if you catch the horizontal bars between stitches. On garter stitch or ribbing, use the backstitch instead. If you are on stockinette but the seam still shows, you may be going too far from the edge — stay within one stitch of the edge on each side.

If your backstitch seam is wavy or uneven, your stitches are too large or too loose. Undo and work with smaller, tighter stitches. If the seam is splitting or the yarn is shredding, your needle is splitting the yarn strands instead of sliding between them. Use a blunt yarn needle, not a sharp sewing needle, and work more slowly.

Seaming different stitch patterns and edges

The mattress stitch works best on stockinette because the horizontal bars are straightforward to see and catch. On garter stitch (knit every row), use the backstitch instead, because garter stitch does not have clear horizontal bars. On ribbing, the backstitch is also the better choice. On reverse stockinette (purl side facing out), the mattress stitch still works, but you catch the bumps instead of the bars — the principle is the same.

If you are joining a live edge to another live edge (both pieces still have stitches on the needle), use the Kitchener stitch instead of the mattress stitch. The Kitchener stitch grafts the two edges together so there is no visible seam at all — the stitches become continuous across the join. This is common when closing the top of a hat or the toe of a sock.

If you are joining a cast-on edge to a bound-off edge, or a cast-on edge to a live edge, the whip stitch is usually the best choice. The whip stitch also works for attaching a flat piece (like a pocket) to the body of a garment, or for sewing a seam that needs to stretch (like the side seam of a sweater in a stretchy yarn).

Frequently Asked Questions

Can I use the mattress stitch on ribbing or garter stitch?

No. The mattress stitch depends on the horizontal bars that appear between stitches on stockinette fabric. Ribbing and garter stitch do not have these clear bars, so the mattress stitch will not produce an invisible seam. Use the backstitch on ribbing and garter stitch instead.

What do I do if I run out of yarn in the middle of a seam?

Weave the end of the old yarn through a few stitches on the inside of the seam and trim it. Thread a new length of yarn onto your needle and weave it into the seam, overlapping the old yarn by at least an inch. Continue seaming. The overlap keeps the seam from separating.

How tight should I pull the seaming yarn?

Pull snug enough that the two pieces draw together and the seam does not gap, but not so tight that the fabric puckers or the yarn breaks. If you are unsure, pull gently and check every few stitches. You can always tighten a loose seam, but you cannot easily loosen a tight one without undoing it.

Should I block my pieces before or after seaming?

Block before seaming if your pattern calls for blocking. Blocked pieces have even edges and sit flat, which makes seaming much easier and produces a neater result. Some knitters block again after seaming to even out the seams, but blocking before is the standard approach.

What is the difference between the whip stitch and the mattress stitch?

The mattress stitch joins two edge stitches together and works only on stockinette. The whip stitch joins a live edge to a bound-off edge or flat surface and works on any stitch pattern. Use the mattress stitch when both pieces have finished edges; use the whip stitch when one piece has live stitches still on the needle.