A seam allowance is the space between a seam and the edge of the fabric. Since sewing a seam right against the edge of two pieces of fabric can lead to fraying and may not hold, it is important to include a seam allowance that ensures that the seam will be sturdy and not run off the raw edge of the fabric. Seam allowances are also useful when making garments or products that may need to be altered, such as clothing.
There are many different sizes of seam allowances; different widths may be appropriate for different types of sewing project. Most clothing patterns use a 5/8ths of one inch (1.58 cm) seam allowance, while quilting blocks usually require a quarter inch (.635 cm) allowance. Most sewing machines come with standard markings that allow a seamstress or tailor to align the edge of the fabric to any of the most common allowance markings. As long as the fabric is held tight and straight, the machine will stitch the seam, leaving the exact allowance required.
Allowances are very important in clothing because they give the seamstress or tailor room to adapt to larger measurements. Making a garment smaller is usually not too big of a problem, as seams can simply be torn out and re-sewn to be tighter. Enlarging a garment, however, is nearly impossible without a seam allowance, because the seams cannot be expanded beyond the raw edge of the fabric without adding new inserts of material. Novices may want to allow for a slightly larger allowance than strictly necessary, in order to make alterations easier.
It is important to sew seam allowances correctly when following a sewing pattern. Most patterns specify the seam allowance needed based on the size and shape of the fabric pieces. If the seam is sewn too close or too far from the edge, the proportions of the garment will be incorrect. It may be difficult to add additional pieces to the project, and fit will likely be seriously affected.
Some people like to finish off allowances with another row of stitching to prevent fraying or create a finished look around the edges. There are many different stitches that can be used to finish an allowance, such as the zig-zag and French seam. A zig-zag setting comes standard on most sewing machines and creates a strong, finished edge in an interesting zig-zag pattern. French seams are often used when the raw edge will be visible on the finished project, and require a second seam to run along the edge on the back or wrong side of the fabric. When the fabric is flipped to the “right” side, and the real seam is shown, the French seam is enclosed within the fabric, thus hidden from view.