Previous work has already modelled an open periodic cylindrical tube constructed from a Frequency-Selective Surface (FSS) to form the Frequency-Selective Guide (FSG). This model is used to expand the understanding of the FSG and known mode content that it can support. The results of the model have been authenticated directly by measurement techniques. The range of FSGs measurements undertaken was expanded to enable greater understanding of the structure utilising parameters that could not be included in the theoretical model. This extensive measurement set combines with the modelled data to provide a very comprehensive understanding of the FSG operation based on both physical and theoretical data. [Continues.]