Inward-cut triangle notches are triangle cut outs that serve as markers to match corresponding seam edges together in the dressmaking process. Triangle notches should be marked on your paper pattern such that they can either be cut outward, as a triangle extension, or inward, as a triangle indent. In the latter case, the triangle cutout should not go past the midway point of the seam allowance since cutting too close to the seamline can weaken the finished seam.
Inward-cut triangle notches are visible and easy to work with in seam alignment process. These notches work well for sewing beginners as they provide an easy and convenient way to find matching seams and align them in the sewing process. Transferring them from your sewing pattern to the corresponding fabric piece(s) is fairly easy to do but as always, you should exercise care when cutting into the seam allowance layers.
Start by pinning your pattern to the fabric layer or layers underneath. Follow conventional pattern pinning and alignment rules. Begin cutting along the pattern's straight edge until you arrive at the very beginning of the triangle notch. At this point, carefully slit into the seam allowance layers, following the notch's triangular lines as a guide. Do not cut past the triangle point or you risk cutting to close to the seamline. The idea of an inward-cut triangle notch is to provide the right amount of visibility along the cut fabric edges without jeopardizing the seam's integrity or limiting the seam finishing techniques you can apply.
As opposed to outward- cut triangle notches, which are cut as extensions and can be trimmed when np longer in use, inward-cut notches leave an irreversible mark within the seam allowance. This is ok if the seam allowance layers will be trimmed down or finished with a secured serging stitch and enclosed on the inside of the garment. However, some special seam finishing techniques (such as a french seam, welt seam or flat felled seam, to name just a few) require smooth, uncut seam allowance layers and thus inward-cut triangle notch may not provide the most efficient way to mark corresponding seams. In this instance, it is best to cut your notches as outward triangle extensions as this will leave seam allowance layers intact and suitable with all finishing techniques.