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.