The GTL 5050 was created specifically to allow probing on opposite sides of a PCB. It allows the probes and measurement system to be setup and calibrated as a traditional "co-planar" probing system. It has a mechanism that allows the input port probe positioners to be clamped directly to the sample. Then the entire sample plus positioners are "flipped over" for positioning of the 2^nd probes/ports. This mechanism allows direct thru measurement of structures like pogo-pin contactors, BGA package I/Os and vias, as well as thru paths of transmission lines that traverse right-angle board-to-board connectors. When used with a differential test instrument, it makes possible direct measurement of differential impedance of these structures.