According to U.S. MIL-STD-1472D, Human Engineering Design Criteria for Military Systems, Equipment, and Facilities, an average man using both hands can safely pick up an object from the floor, carry it, and place it on a horizontal surface 5 ft (1.52 m) high if it weighs no more than 56 lb (25.4 kg). If we assume a cylindrical plane-parallel solid mirror of approximately 9:1 diameter-to-thickness ratio made of ultra low expansion (ULE), this weight would correspond to a diameter of ~20.0 in. (~51 cm) and a thickness of ~2.23 in. (~5.66 cm). Although arbitrary, this calculation forms the basis for the size boundary used here to delineate between a small and a large mirror. Designs for single-substrate mirrors as large as ~8 m (~26 ft)—for which major considerations are ways to reduce weights and surface deflections to tolerable levels—are discussed in Chapter 2 of Volume 2. Techniques for mounting large mirrors in different orientations relative to gravity are considered in Chapters 3 through 5 of Volume 2. Chapter 2 of Volume 2 also includes some considerations of segmented-aperture mirror arrays of even larger sizes. The design and mounting of metallic mirrors of various sizes are the subjects of Chapter 6 of Volume 2.