Abstract: Apparatus for ensuring the proper interconnection of an electrical component (11) and the terminal pins (15) of a backplane (10). A keying member (14) having a pair of dowels (26, 27) at a predetermined spacing uniquely selected from a plurality of possible spacings (a.sub.1 -a.sub.4 ; b.sub.1 -b.sub.4) is associated with an array of pins (15) to which connection is to be made. The component (11) presents a vertical plate, the leading edge of which is notched (40, 42) to correspond to the dowel spacing so that, as the component is fitted to the pin array, the dowels (26, 27) freely enter the notches (40, 42). A connector block (34) and a stiffener member (38) on opposite sides of the plate are fluted (43, 44) to correspond to the plurality of possible spacings and to present possible cylindrical holes thereat, completed cylindrical holes being presented only where the plate is correspondingly notched. Polarization is achieved by avoiding a symmetrical spacing of the dowels along the keying member.