sufficient to cause electrical shock, the semi-exposed center contact of the female plug would be considered unacceptably hazardous, requiring a different choice of power connector. Ambiguous gender Some electrical connectors are hermaphroditic because they include both male and female elements in a single unit intended to interconnect freely, without regard for gender. See the discussion of Genderless connectors elsewhere in this article for more detailed information. As an additional complication, certain electronic connector designs may incorporate combinations of male and female pins in a single connector body, for mating with a complementary connector with opposite gender pins in corresponding positions. In these unusual cases, gender is often defined by the shape of the connector body, rather than the mixed-gender connector pins and sockets. These types of connectors are not strictly speaking hermaphroditic, since mating connectors are not freely interchangeable. An informal term that has been used for these connectors is "bisexual", in addition to the more official terminology, mixed-gender. Thus, for example, one can have a mixed-gender female plug that connects to a mixed-gender male jack (though a reversed gender assignment of connectors would be a more typical design choice in this example). Male connector pins are often protected by a shell (also called a shroud, surround, or shield), which may envelop the entire female connector when mated. RF connectors often have multiple layers of interlocking shells to properly connect the shields of coaxial and triaxial cable. In such cases, the gender is assigned based on the innermost connecting point. With the exception of reverse polarity BNC or TNC, where the outer shell determines the gender and the innermost connecting points are opposite to a standard connector, for example a female RP-TNC connector has a solid innermost pin. Another ambiguous situation arises with the connectors used for USB, FireWire (IEEE-1394), HDMI, and Thunderbolt serial data bus connections. Close examination of these connectors reveals that the contact "pins" are not actually pins, but instead are conductive surfaces that slide past each other when they mate. Therefore, the traditional pin and socket nomenclature is not applicable. Instead, most computer hardware people fall back to referring to the wrap-around metal shield on the plug connector as if it were a connector pin. By this convention, the connectors on serial bus cables are "male plugs", and the corresponding connectors on equipment are "female jacks". A casual glance at a USB "Type A" plug connector