In the past 30 years, technology advances have come not just in materials like aramids and carbon but also in how those fibers are arranged in a two-dimensional fabric. For example, knitted fabrics differ from woven roving, whose filaments of equal diameter cross at 90-dgree angles. In knits, the main or structural fibers are stitched together with fine nonstructural filaments, usually polyester. The structural fibers lie flat, not crimped, leaving less space for resin to fill. With knits, builders can get a lot more strength and stiffness per unit thickness.