In the apparel sewing process, you will come across a few different style of patterns which will dictate the way to mark and cut your garment pieces. If you have some experience sewing, you already know that you cannot sew a seam without seam allowance. Most comercial sewing patterns should provide a stitch line or a seamline and a cut edge. The distance from the stitch line t the cut edge is by design, your seam allowance. In some instance however, you may come across patterns to not include pre-marked seam allowance, requiring you to add at the time of cutting out your garment pieces. These patterns are used by more advanced dressmakers as they provide a way to add the desired seam allowance along each edge, base don desired seam finishing and hem finishing techniques used.
Note: The most common types of patterns that do not have pre-marked seam allowance are called slopers. Slopers or blocks are used at the manufacturing level as a staple or building block to generate other designs and sizes. The seam allowance is added at the end. once the new sewing patterns have been generated from the slopers. The slopers serve as reusable stencil and are never altered. New patterns are always generated from sloper copies.
If you find yourself in a situation where your sewing patterns do not have pre-marked seam allowance, you can mark the desired seam allowance using a ruler and tailor's chalk. Start by aligning the pattern to the fabric perfectly on grain- the pattern's grainline should be parallel to the fabric selvage edge. If you are not able to pin through the pattern and fabric layer(s) underneath, use fabric weights (or something similar) to keep the pattern flat and aligned on grain. Trace carefully along the paper pattern's edge using tailor's chalk or a fabric pencil. The lines left behind should be the exact replica of the paper pattern. These initial trace lines will mark the seamlines or stitch lines. Before cutting this garment pieces, seam allowance has to be added directly on the fabric as shown below:
If you are working from a pattern set that has instructions, or you drafted the patterns yourself, you should know exactly what seam allowance width to add along all edges. In this case, all edges have 1/2" seam allowance and have been marked with a clear plastic ruler and tailors chalk. Marking the seam allowance will naturally generate the pattern's final cut lines. Following the outer lines, cut the fabric piece(s) carefully and repeat the marking and cutting process on the remaining garment patterns.
Note: It is important that when marking your seamlines directly on the garment's fabric pieces, you use marking tools that are designed for fabric and easy to remove in the wash without staining. Tailor's chalk works great on darker, textured fabrics, while a fabric/quilting pencil offers more precision and works best on smoother surfaces and thinner fabrics.