We go through great length to make sure our garments are as comfortable as possible. Therefore we apply a special seam to close the elastic band on our underwear. This requires a very special machine, but it allows us to connect the elastic band with a strong and very flat seam. This looks better and is more comfortable.
The cheapest alternative solution is to overlap the elastic band and connect it with a double seam. This is cheap because production is fast and it does not require a special machine, but it is uncomfortable and it looks awful. A solution that is also often applied is the double fold seam. It is applied because it is the easiest way to make a strong seam that looks good. It requires a special machine, but since this kind of machine can be used for many other seams it is relatively cheap. It is a strong seam that looks good, but it is quite thick and therefore uncomfortable.
Our solution is a flat end-to-end seam. Making a flat seam as such is not rocket science, but making one that is strong enough for application on elastic band is. Therefore it requires a special machine that can only make flat end-to-end seams for elastic bands.