Triangle notches are the best to use in the dressmaking process for sewing beginners. If you take a look at a sewing pattern you will notice that the notches are marked with a triangle shape that allows you to either cut the notches inward or outward. Choosing to cut the notches outward will provide a visible triangle extension that serves as a way to find corresponding seam edges and align them in the sewing process.
Outward cut triangle notches are fairly easy to transfer from pattern to fabric and can be conveniently shaved off when no longer needed (usually after the seam has been aligned and stitches). The ability to remove them without affecting the seam allowance layers renders them the safest to use in the garment construction process as well as the most versatile, having no negative effect on the garment's seam finishing techniques.
To transferred outward-cut triangle notches: Pin your sewing pattern to the corresponding fabric layer(s) and cut along your pattern edges carefully. When arriving at the triangle notch extension, cut around the the notching leaving some fabric excess. This will provide added comfort when slashing next to each triangle edge.
Slash next to each triangle edge, being careful not to slit into the sewing pattern.
Cutting the perfect triangle notch will take a few tries, as long as the resulting triangle extension is visible along the resulting fabric edges, this should be enough to get the job done in the garment alignment and sewing process. Once the notch is marked continue cutting your sewing pattern as needed. Repeat the process when/if arriving at the next triangle extension notch. Follow the same logic when transferring double or triple notches.
The resulting triangle notches should be visible and provide a clear and convenient way to match corresponding seam edges. When looking at your garment pieces, it is helpful to align the fabric edges next to one another based on corresponding notches- this will keep you organized and speed up the alignment and stitching process.
As noted above, the beauty of extension notches is not only their visibility, but also the ability to safely remove them when no longer needed. When the seams are aligned and stitched and you are gearing up to apply your seam finishes, it may be helpful to slash the triangle extensions flush with the fabric edge (seam allowance edge). Doing so should give you a clean canvas and apply your preferred seam finishing techniques without jeopardizing the smoothness and durability of the seam allowance layers.