Abstract: A sign assembly can be formed by attaching several modules together. Each module includes a lamp assembly for back lighting the sign display, and has walls with a plurality of dovetail grooves in their exterior surfaces. Double dovetail unions engage the dovetail grooves in abutting walls of a pair of modules to fasten those modules together. Other dovetail unions fill in exposed grooves or fasten a frame around the perimeter of the sign assembly. Each module has a wiring assembly with connectors at each corner so that adjoining modules can be electrically interconnected. This enables electricity to be externally applied to only one module of the assembly with the other modules receiving electricity directly or indirectly from that one module.