Understanding Interfacing Patterns
If you are working with a store-bought pattern set, you may notice a set of unique sewing patterns that are designated for cutting the garment's interfacing. While not visible ion the garment's wrong side, interfacing is added along key portions of the garment to add structure and stability. These ares include a garment's neckline, armholes, hem, cuffs, collars, some pockets and waistband.
Interfacing patterns are easy to identify as they usually feature no seam allowance. They are generated from the portions on the garment that require the interfacing and thus should be the identical replica of these area's designated patterns minus the seam allowance. When using these interfacing patterns to cut fusible interfacing, you should be mindful of the pattern's designated face side. The pattern's face side should be aligned such that it matches the interfacing's adhesive side which should correspond to the garment's wrong side at the interfacing's designated areas.
Keep in mind that some pattern sets do not include separate interfacing patterns even if interfacing is required in the garment. In this case, you can use the garment's pattern pieces to cut the separate interfacing pieces. This is often the case when interfacing smaller areas such as cuffs, collars and facings.
To minimize bulk, it is standard practice to trim the seam allowance off the interfacing pieces before fusing them to the garment. If using sew-on interfacing, you may have to leave the seam allowance edges untrimmed until after the sew-on interfacing is attached. However, almost all interfacing patterns (if included in your pattern set) are designed for use with fusible interfacing and thus should not include seam allowance, fitting within the garment's seam-lines without adding bulk into the garment's seam allowance areas.