Seam allowance is a little strip of cloth between the sewing line and the very edge of the material. It’s tiny, so people usually overlook it as something minor. When sewing clothes, seam allowance determines size, durability, edges, and how good a seam can be pressed. If you sew seam allowance in some sections differently from others, the fabric pieces may not fit as the pattern intended.
Use two scraps of material with two chalk lines on each, and follow the two lines. Sew the seam on both pieces so that both seam allowances are the same, and sew them on the stitching line. Sew one seam to make the seam allowance wider on one side, and the narrower seam allowance on the second pair. Open the seams and look at them. The second seam will be more uneven than the first. It’s easy to see why seam allowance is important, and not just a strip of cloth.
Patterns typically give you a standard seam allowance, and this needs to be accurate. For instance, if your seam is to be at 1.5 cm allowance, then when you sew the seam at 2 cm allowance, the garment gets too small at the side seam. Another seam at 1 cm will also change the shape again. A single uneven seam will be fine on a practice piece, but multiple uneven seams can affect the whole garment, like the waistline, armholes, hems, and the sides (left vs. right).
The machine may help you but only if you pay attention. The sewing machines have some guide marks on the sewing plate. Find the mark that is equal to your seam allowance, and keep the raw edge of the fabric moving along that guide. Do not look only at the stitch. Look at the edge of the fabric, to make sure that the allowance is still in line. Guide the fabric with the machine under the foot; do not drag or pull the fabric from underneath. Pulling will pull away the allowance from the line, or make the fabric stretch out.
The curves and corners can be tricky too. The raw edge will move on one part of a curved line, like on an armhole or neckline facing. Move the fabric slowly so that you can keep the allowance in one place as you make the curves. The corners are also a problem. At the corner, stop the sewing with the needle inside, raise the foot, turn the fabric and put the foot back again. This way the seam allowance does not get wider or too narrow.
It also happens to the finishing. An edge which is too thin can be difficult to finish by zigzag, overcasting, trimming, or pressing. A wide seam can be too bulky on hems, elastic casings, waistbands or facings. Press the seams after sewing. Check how the edge will lie on the pattern. Trim any raw edge after this is done. Once fabric is cut away, it is harder to correct. Make sure everything before you start trimming.
Before you sew your garment seams, take a moment to consider: do I know exactly where the stitching line should be? If you are not sure, put a chalk mark or line on the fabric to show the seam allowance. Check the mark on the pattern. If you are unsure, make a practice seam on a piece of cloth and use the same allowance on the rest of the piece of cloth. Clean seam allowance does not require quick sewing. Sewing clean seam allowance comes with practice, by making sure that the edge is straight along the line, checking the seam allowance, and making sure that everything is still in order before sewing the whole piece.