A desirable sign structure can be formed using a hollow, integral, self-supporting thermoplastic polymer body shaped so as to include a hollow border or peripheral frame carrying front and back walls of the sign structure. Grooves adapted to carry extensions on interchangeable letters may be provided in the front wall. When such grooves are used, the back wall preferably includes grooves or furrows reinforcing the front wall. A slot can be provided in the front and back walls in such a manner as to separate the front and back walls into two panels or panel sections. When such a slot is used, guides or tracks are preferably employed so as to mount a shutter in the slots, the shutter being capable of movement in order to cover or uncover one of the panels or panel sections.