OptiMA's broad line of enclosed letter boards and/or changeable letter boards will attractively display directory listings, messages, notices and directions in schools, businesses and institutions alike. Our changeable letter boards are made with a grooved felt letter system for indoor units and a grooved vinyl material for outdoor enclosed units. These locking changeable letter boards are designed for durability and stability in high volume, high traffic areas. Some units come with a set of plastic letters and other changeable plastic letters are available separately for purchase. Most of the indoor enclosed letter boards use tempered safety glass in the windows. The outdoor changeable letter boards use a shatter resistant acrylic window. All of the units have locks on the doors.