You can cut two identical fabric pieces from the same pattern and sew them into an accessory that ends up looking skewed or too small. As you stitch, if the seam allowance gets closer to the raw edge on one part of the piece, that part becomes larger. If it goes further away, that part becomes smaller. For a straight-sided bag, this might cause the sides to shift or the lining to fit incorrectly. For items with curved edges, boxed corners, or zipper flaps, inconsistent seam allowance has a much greater effect, because multiple seams are joining together.
The seam allowance is the distance between the seam and the edge of the fabric. Keeping the seam allowance consistent keeps the proportions of the pattern intact. If your pattern specifies a 1 cm seam allowance, but you sew some of the seam at 0.7 cm and others at 1.3 cm, you change the shape. Even though it looks like a small change at first, if you’re joining two pieces together along the top, bottom, and then attaching a lining, you’ll end up with two sides that don’t quite line up.
Try this: take two scraps of fabric, cut them into strips, and mark a seam allowance along one edge. Place the strips right sides together and pin or clip them. Sew the seam allowance as slowly as you can, aligning the raw edge with a consistent point on your machine. Measure the seam allowance after you sew, noting how far away from the raw edge the seam is at several points along the length. If the measurement doesn’t stay consistent, try again, but focus your eyes on the fabric edge and the seam allowance line on your machine instead of looking at the needle.
When you sew a curve, be sure to avoid pulling the fabric too much to get around the needle. Doing so stretches the fabric edge, which alters both the seam allowance and the shape. It’s best to sew a little bit, stop with the needle down, raise the presser foot and realign the fabric, then continue. For very tight curves, sewing more slowly allows you to maintain control. Clipping or notching the seam allowance (after the curve is sewn) allows the fabric to lay flat without altering the stitching line.
Inconsistent seam allowances also make themselves obvious in corners. Let’s say you are sewing the side and bottom seams of a small box. If one side seam allowance is further in than the other, the corners won’t intersect evenly. You should be able to tell if there’s a difference before you flip the bag right side out, by looking at the seams and how they meet. You can adjust these before trimming and clipping the corners; once you’ve trimmed the seam allowance, it’s not as easy to fix.
You don’t need perfectly consistent sewing for your accessory to look good. The important thing is to recognize when your seam allowance is shifting and to make an adjustment before it gets too far off. A smooth fabric, a consistent sewing guide, proper pattern cutting, and patience will help you achieve good results. If your finished bag, wallet, or organizer seems a bit wonky or your lining fits tighter than it should, double-check your seam allowance. It might just be the reason why the shape has changed.