Abstract: The subject key block arrangement eliminates the need for modifying connectors or circuit boards and functions to prevent a connector having a width greater than a predetermined size from being inserted into certain positions in the circuit pack carrier. This is accomplished by an L-shaped key block which is attached to the backplane of the carrier. The upright portion of the key block in combination with the existing card guides on the carrier present a predefined aperture into which only connectors of a matching size will fit. Connectors of greater width are blocked from being inserted into connector positions equipped with the key block. Thus, standard connectors, circuit boards and carriers can be used without modification. The key block is inexpensive to manufacture and requires only minimum labor to install in an existing carrier.