This chapter describes the derivation of an analytical model for the radiated emissions from an enclosure with three closed and one open edge depicted in Figure 7.1. A trace above the ground plane of the PCB couples to the cavity and causes radiation from the slot of the enclosure. Intended applications of this model are fast predesign investigations on the influences of enclosure dimensions, placement of potentially critical components, and the PCB layer stack. Especially the first resonances of parallel-plane structures depend mainly on the maximum overall enclosure dimensions in x and y direction and not so much on details of the enclosure shape. Therefore, the model provides good first order information even about quantitative emission values for practical enclosures. The in depth description of the model derivation in this chapter shall provide guidance for the derivation of such enclosure models with different boundaries, for example, an enclosure with connector slots on the front and the back edges.