Flathead pins provide the most versatility in dressmaking. While they are arguably not always the most comfortable to work with, their small size and dainty shape provides a huge advantage when draping on a dress form or pinning a large number of folds and pleats.
Flathead pins are usually no longer than 1.5 inches in length, featuring a subtle flattened area on the opposite end of the needle point. Their unique shape and smaller length is quite useful when fitting a garment from its face side- this unassuming shape is able to visually disappear into a seam or textured fabric surface allowing for a more accurate garment fitting and shaping process.
As noted above, flathead pins are commonly used when securing seams from the garment's face side. Given that they are not as visually noticeable, flathead pins provide a flat and smooth way to imitate a seam. If you are unsure whether to use flathead pins or ball-head pins with you dressmaking projects, consider what time of work you'll be doing:
If you plan on designing and draping on a dress form, flat head pins will provide the best way to recreate smooth, realistic seams, folds, pleats and style lines. In this instance, there are no bulky additions on the opposite end of the needle point (like T-shape and ball-head pins) resulting in a more precise and accurate dressmaking process.
If you are mainly working from a sewing pattern and using pins to align fabric edge together prior to machine stitching, flathead pins may be a bit uncomfortable to work with since they don't provide a fast, secure grip in the insertion and removal process. In this case, universal ball-head pins will work much better. Nevertheless, as you become more advanced, it is a good idea to keep a pack of flathead pins handy as you never know when you may need them for a special draping or fitting task.