How to Seam Knitting and Crochet

Block first. This is the whole trick

Seaming two curling, uneven pieces is difficult. Seaming two flat, correctly-sized pieces is straightforward, and the difference is one wash and a night on a mat.

Blocking also tells you whether the pieces are actually the same length before you sew them together, which is considerably better than finding out afterwards. Our how to block your knitting page has the method.

The four methods

Mattress stitch, step by step

The one worth learning. It joins two vertical edges from the right side, and done properly the seam disappears completely into the fabric.

1. Lay both pieces right side up, edges together. Not overlapping, and not right sides facing.
2. Find the running bars. One stitch in from the edge, between two columns of stitches, there is a horizontal bar per row. That is what you sew through.
3. Thread a long tail on a blunt tapestry needle. Use the cast-on tail if it is long enough.
4. Go under one bar on the left piece, then one bar on the right. Alternate, working upward.
5. Do two bars at a time once you are confident - it is faster and equally invisible.
6. Pull gently every inch or so. The seam closes and the stitches interlock. Do not pull hard or the seam puckers.

Matching row for row

The commonest seaming failure is a side seam that ends up longer on one side than the other, because the two pieces drifted apart by a few rows.

1. Count the rows on both pieces first. They should match; if they do not, ease the difference over the whole seam rather than at the end.
2. Pin every ten rows before you start, with locking markers, on both pieces. It takes two minutes and it is what keeps the seam honest.
3. Check as you go. Every few inches, look at whether the markers are still aligned.
4. Ease, do not gather. If one piece is two rows longer over 60 rows, take two double-bars on the longer side. Spread over the seam, it is invisible.

Shoulders, which want a different method

Shoulder seams carry the weight of the whole garment, so they want structure rather than invisibility.

The three-needle bind-off joins two sets of live stitches by knitting them together and binding off in one pass, leaving a neat internal ridge that acts as a structural spine. It is faster than sewing and stronger.

If the shoulders are already bound off, a backstitch seam does the same job. Do not use mattress stitch on a shoulder - it is invisible and it stretches.

Crochet seams

Fast, and they show. That is not always a fault.

• Slip stitch join. A firm, low ridge. Good for blankets and anywhere you want a defined edge.
• Single crochet join. A bolder ridge that becomes part of the design. Standard on granny square blankets.
• Join-as-you-go. Attaches each motif during its final round, which removes both the seaming session and most of the ends.
• Whip stitch with a tapestry needle is the flattest crochet join, and the right one for anything that has to sit against skin.

Which yarn to seam with

The same yarn, in almost every case - it behaves and washes identically, and it disappears.

• Bulky or chenille: split the yarn, or use a smooth thinner yarn in the same color. A bulky seam in bulky yarn is a rope.
• Textured or novelty yarn: use a smooth yarn in a matching shade. Textured yarn will not pull through cleanly.
• Anything that will take strain: consider adding a fine matching thread alongside for strength.
• Never a contrast color, unless it is deliberate - it will show at every stitch.

Seamed or seamless?

Worth knowing that seaming is a choice rather than an obligation, and that the fashion for seamless construction is not straightforwardly an improvement.

For a heavy garment, seams are a genuine structural advantage. For a lightweight one, seamless is less work. Neither is the correct answer in general.