How do I sew up a knitted jumper so the seam does not show?

Most home-knitted garments are let down at the seaming rather than the knitting. Sewing up is a separate skill, it is done from the right side so you can see what you are making, and the one technique that matters is mattress stitch — everything else is order of assembly.

What you'll need

• A blunt tapestry or darning needle
• The same yarn, or a smooth yarn of the same colour
• Pins with large heads
• A flat surface bigger than the pieces

Step by step

1. Block the pieces before you sew anything.Pin each piece out to the measurements on the pattern schematic and either steam it or wet it and let it dry flat. Blocking evens the stitches, makes stocking stitch stop curling, and turns two edges that did not match into two that do. Skipping it is why seams fight back.
2. Lay the pieces out and check they match.Front against back, sleeve against sleeve. If one is longer, decide now whether to ease it in or re-block. Discovering it while sewing means unpicking.
3. Use a smooth yarn if the garment yarn is textured.Mohair, boucle and thick-thin yarns are almost impossible to unpick and hard to tension. A smooth yarn in the same colour seams better and is invisible in the finished seam.
4. Work from the right side, both pieces face up.Lay them side by side, right sides towards you, with the edges to be joined next to each other. Seaming from the wrong side is guesswork; from the right side you can see the seam closing as you go.
5. Find the ladder one full stitch in from the edge.Between the outermost column of stitches and the next one in, there is a series of small horizontal bars. That ladder is what you sew into. The outermost stitches themselves are the loosest and most distorted in the fabric and must be left inside the seam.
6. Take two bars at a time, alternating sides.Needle under two bars on the left piece, across to the right piece, under two bars, back to the left. Always work upwards and always enter in the same ladder.
7. Tension every five or six rungs.Pull the seaming yarn gently until the two edges roll together and the thread disappears. Stop there. Pulling harder gathers the seam and makes it shorter than the pieces on either side of it.
8. Seam in the right order.For a typical drop-shoulder jumper: shoulder seams first, then set the sleeve head into the armhole, then side and sleeve seams in one continuous run from cuff to hem. Doing sides before shoulders makes the armhole impossible to reach.
9. Join stitches to rows carefully at the armhole.You are matching a row edge to a stitch edge, so the counts do not correspond. Pin at the shoulder point and at intervals, and distribute the difference evenly rather than easing it all in at the end.
10. Fasten off inside the seam.A few backstitches into the seam allowance, then weave the tail along the seam for 5cm. No knots.
11. Press the seams lightly if the fibre allows.Wool takes steam well; acrylic will flatten and go limp permanently under a hot iron. Check the ball band and use a cloth.

Tips

• Pin at the top, the bottom and the midpoint of a long seam before starting, so any difference in length is distributed rather than arriving all at the end.
• Good light. Finding the ladder is a visual job and it is nearly impossible in a dim room.
• Leave long tails when casting on and off — 40cm gives you enough to seam with, and one fewer end to weave in.

Common mistakes

• Sewing through the outermost stitch — Those loops are the loosest in the fabric and they distort. Sewing through them produces a lumpy ridge on the right side and loses a stitch of width from each piece.
• Skipping blocking — Unblocked stocking stitch curls hard at the edges, so you cannot see the ladder and cannot lay the pieces flat. Everything about seaming is harder, and the finished garment still needs blocking anyway.

If it doesn't work

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