Seaming knitted pieces uses a needle, thread, and one of three main stitches

The most common way to join knitted pieces is the mattress stitch, which creates an invisible seam by threading a needle under the loops along the edges of your pieces. You thread horizontally between the two pieces rather than sewing them face-to-face, so the seam sits inside and the knit pattern lines up perfectly on the outside. The other two methods—backstitch and whip stitch—are visible and faster, but mattress stitch is what most patterns call for because it looks professional and doesn't add bulk.

All three methods use the same basic tools: a tapestry needle (blunt, with a large eye), yarn that matches your knit, and the two pieces you want to join. The pieces should be lying flat in front of you, edges touching but not overlapping. You'll thread the needle under specific loops in a repeating pattern, pulling gently to close the gap as you go.

Key Takeaways

  • Mattress stitch is invisible because you thread the needle horizontally between the two pieces rather than sewing them together face-to-face.
  • You need a tapestry needle (blunt tip, large eye), yarn that matches your knit, and pieces laid flat with edges touching but not overlapping.
  • Mattress stitch works by threading under the horizontal bars between stitches on one side, then the other, in a repeating pattern.
  • Backstitch and whip stitch are faster but create visible seams and are used mainly for seaming pieces at right angles or attaching ribbing.

How to do mattress stitch step by step

Lay your two pieces flat on a table with the right sides facing up and the edges you want to join touching but not overlapping. Thread your tapestry needle with a length of yarn about 18 inches long (you can always thread more). 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 horizontal bar between the first and second stitch on the left piece, going from bottom to top. Pull the yarn through, leaving a small tail. Now insert the needle under the corresponding bar on the right piece, again from bottom to top. Pull gently. Go back to the left piece and insert under the next bar up, then back to the right piece and the next bar up. Keep alternating sides, working your way up the seam.

Every few rows, pull the yarn snug to close the gap between the pieces. The seam will disappear as you tighten it. Do not pull so hard that you distort the stitches—just firm enough that the edges meet. When you reach the end, weave the needle through a few stitches on the inside to find it, then trim the yarn.

Finding the right bars to thread under

The horizontal bars you thread under are the threads that run between columns of stitches. If you look at a knitted piece from the side, you'll see vertical columns of V-shaped stitches. Between each column and the next is a small horizontal thread. That thread is what you're after.

On a stockinette stitch piece (the most common), these bars are straightforward to see. On textured stitches like garter or ribbing, the bars are less obvious but still there—you may need to use your needle to gently separate the stitches and find them. If you're seaming two pieces that were cast on or bound off, you'll thread through the actual cast-on or bind-off loops instead of the bars between stitches.

Backstitch for seams that don't need to be invisible

Backstitch is faster than mattress stitch and works well when the seam will be hidden—for example, the side seams of a sweater that will be worn with the seam on the inside. Lay your pieces with right sides facing each other (not right sides up). Thread your needle and insert it through both layers at once, about one stitch in from the edge.

Bring the needle back out one stitch forward, then insert it again at the point where you just came out. This creates a line of overlapping stitches that hold the pieces together. Work your way along the seam, keeping your stitches small and even. Backstitch is visible and adds a small amount of bulk, so it's not ideal for seams that show, but it's sturdy and quick.

Whip stitch for attaching ribbing and edges

Whip stitch is used mainly to attach ribbing to the body of a sweater or to join two pieces where one edge is a bind-off and the other is a live edge. Lay the pieces with right sides up. Thread your needle under a loop on one piece, then under a loop on the other piece, working your way along. The needle stays on the surface rather than going deep into the fabric, so the stitch is visible but thin.

Whip stitch is gentler on delicate yarns and works well when you need flexibility in the seam—for example, when attaching a stretchy ribbed cuff. It's also the method to use if one of your pieces is still on the needle (live stitches) rather than bound off.

Common mistakes and how to avoid them

The most common mistake is pulling the seaming yarn too tight, which distorts the stitches and makes the seam pucker. Pull just firmly enough to close the gap. If you've already pulled too tight, you can loosen it by gently working the needle backward and redistributing the tension.

Another mistake is threading under the wrong bar or loop, which creates a seam that doesn't line up or looks lumpy. Before you start, trace your needle under a few bars to make sure you understand where they are. On your first seam, work slowly and check your progress every few rows.

Mismatched yarn color is also common. If your seaming yarn is darker or lighter than your knit, the seam will show. Use yarn from the same ball or skein you knitted with, or buy a matching color from the same dye lot. If you're seaming pieces in different colors, use a neutral gray or tan that blends with both.

Seaming different stitch patterns and edges

Stockinette stitch is the easiest to seam because the bars are clear and the edges are neat. Garter stitch (knit every row) has bumps instead of bars, so you'll thread under the bumps instead. Ribbed pieces have a similar pattern but with more texture—work slowly and use your needle to separate the stitches so you can see what you're threading under.

If you're seaming a cast-on edge to a bind-off edge, thread under the actual loops of the cast-on and bind-off rather than the bars between stitches. If you're seaming two side edges (selvage edges), the bars are still there but may be less obvious because of how the edge stitches sit. Again, use your needle to gently separate and find them.

Frequently Asked Questions

What kind of needle do I use for seaming?

Use a tapestry needle, which has a blunt tip and a large eye. The blunt tip slides between stitches without splitting the yarn, and the large eye holds thicker seaming yarn easily. Tapestry needles come in sizes 13 to 28; size 16 or 18 works for most yarns.

Can I use the same yarn I knitted with, or do I need special seaming yarn?

Use the same yarn you knitted with if possible—it matches the color and texture perfectly. If that yarn is too thick or textured to thread through a needle easily, use a thinner yarn in the same color, or buy seaming thread made for knitting. Never use regular sewing thread, which is too thin and will break.

How much yarn do I need for seaming?

A length of about 18 inches is enough for most seams. If you run out, weave the needle through a few stitches to anchor it, trim, and thread a new length. For very long seams like the full length of a sweater side, you may need 3 to 4 feet total, but it's easier to work in shorter sections and anchor as you go.

What if my mattress stitch seam is lumpy or uneven?

Uneven tension is the usual cause. Pull the seaming yarn more firmly and evenly as you work. If the seam is already done and lumpy, you can carefully undo it by pulling the seaming yarn out and re-seaming. It's tedious but worth it if the seam is visible.

Can I machine-sew knitted pieces instead of hand-seaming?

You can, but hand-seaming gives better results because you can see exactly where you're stitching and adjust as you go. Machine seams on knit can look stiff and are harder to undo if something goes wrong. Hand-seaming takes longer but is the standard for finished-looking knitwear.