This document covers shielded and unshielded free and fixed connectors, circular or rectangular, for balanced single-pair data transmission, with current-carrying capacity. It also covers the portion for balanced single-pair data transmission of combined, shielded or unshielded, free and fixed connectors, circular or rectangular, having additional contacts for power transmission, whose performance requirements are described in a detail specification of the IEC 63171-X series, (type X connectors), or in a separate document, either IEC detail specification or manufacturer s specification. It specifies the IEC 63171 series or of other document referencing it common mechanical, electrical and transmission characteristics and environmental requirements, as well as required test specifications. This document does not describe a specific mating interface. Detail specifications of mating interfaces complying with this document can be found in the family of detail specification standards IEC 63171-X (type X connectors) or in a separate document, either IEC detail specification or manufacturer s specification. Within their own type, the shielded and unshielded connectors are interoperable for their transmission performance and can be exchanged, though the shielded version has improved alien crosstalk and coupling attenuation properties. Single-pair connectors of this series might be grouped to one body of multipole connectors or combined with other connectors, e.g., power connectors, also known as combined (data or signal, and power) connectors.