Prisms are solid geometric shapes that have two identical and parallel bases. There are six main types of prisms defined by the shape of their bases. Prisms can be regular if their cross-sections are regular polygons, or irregular if their cross-sections are irregular shapes. The surface area and volume of prisms can be calculated using formulas that involve the base area, perimeter or base, and height. A frustum is the portion of a prism cut by a plane that is not parallel to the bases, forming a new cross-sectional shape between the cut plane and one base.