Abstract: An advertising sign structure made of a single-piece appropriately shaped open shell of plastic material provided with a continuous lateral flange portion disposed intermediate the bottom and top of the shell for fastening to a backing panel, a resilient gasket being disposed between the marginal surface of the backing panel and the flange surface. The surface of the backing panel disposed within the shell may be provided with an appropriate lighting element such as a contoured tubular fluorescent lamp. In normal use the backing panel is attached to an appropriate support structure, such as posts or a building. The structure consisting of the shell with attached backing panel forms the complete advertising sign, or alternatively, forms an element of the sign such as a single letter or a plurality of letters.