A 12-unit (12U) movable amp rack is a specialized case or cabinet designed to house, protect, and transport audio equipment like power amplifiers, signal processors, and other standard 19-inch rack-mountable gear. The "12U" designation refers to the rack space, with "U" or "Rack Unit" being a standard measurement of 1.75 inches. Therefore, a 12U rack can accommodate 12 x 1.75 inches, or 21 inches, of vertical equipment space.