To get a good fit it's important to have a unit that follows the shape of your head. In order to get that shape correctly the factory used two techniques to 'shape' the base. If there is a slight amount of curvature the factory will put in horizontal stitching to give the base contour. If the shape requires even more shape the factory will put darts in the base. Here's an example of a unit that has both darts and horizontal stitching in order to achieve the exact shape of the template that was submitted.

Hey John.

That's a nice innovation. Does the factory implement these contouring darts on full caps? I was thinking of the nape issue that concern full-cappers. I used a hairpiece company that used similar darts for the front hairline. I didn't need to glue anything down to get a nice hairline.

I wonder if these darts might help with the nape area staying down?

Hi Knockohoma,
Yes, the factory does this on full caps. It's even more necessary to do this on full caps since there's more contour needed to properly shape a full cap. The larger the unit the more shaping is necessary. Some smaller pieces need no contouring at at---the area is small enough that the lace just lies flat on the bald area. Some units need a slight contour and the factory doesn't need to do darts---just the horizontal stitching will be enough to add the necessary shape to the unit. And then again some units need more shaping and the factory will do darts. Some companies will provide a unit based just on dimensions. Sometimes this will work. But beware of companies that just ask for dimensions---they won't get the custom fit that a unit based on a template will give you. To get a proper custom fit making a template is the best way to go to ensure that the contour matches your head shape.
Regards,
John

Good info. I would love to see more informative posts of this sort John. Keep 'em coming!