Every pattern piece is drafted so that its seam lines match when sewn at the stated allowance. Sew one seam narrower and that piece becomes longer than the one it joins, and the error accumulates around the garment until a sleeve does not fit an armhole. The fix is not precision, it is consistency: pick a guide, use the same guide on every seam, and never watch the needle — the needle is where the stitch already happened.
Questions people actually ask
What seam allowance should I use?
Whatever your pattern specifies — commonly 15 mm for garments, 6 mm for quilting. What matters more is using the same one throughout, because the pattern pieces were drafted to match at that width.
How do I keep it consistent?
Watch the fabric edge against the guide line on the throat plate, not the needle. If your machine has no useful markings, stick a strip of masking tape at the right distance and follow that.
Last reviewed 2026-08-11