Abstract: A series of electrical connector assemblies each comprising plugs and corresponding sockets which are of a number of different widths. Narrower plugs of the series are prevented from insertion into wider sockets of the series by the variation of the height of the plugs and sockets and or the cross-sectional dimensions of a key and keyway provided on the plugs and sockets. Further differentiation, between plugs and sockets of the same width, is achieved by variation of the location of the key and keyway on the plug and socket sidewalls and or the cross-sectional dimensions of the keys and keyways.