When used for POTS, the first wire is known as the "tip" or "A"-leg (U.K.) conductor and is usually connected to the positive side of a direct current (DC) circuit, while the second wire is known as the "ring" lead or "B"-leg (U.K.), and is connected to the negative side of the circuit. Neither of these two sides of the line has a connection to the local ground. This creates a balanced audio circuit with common-mode rejection, also known as a differential pair. The tip and ring convention is based on the ″ (6.5 mm) TRS phone connectors, which were employed in telephone switchboards in the 19th and 20th centuries, where the tip contact of the connector is separated from the ring contact by a spacer of insulation. The connection furthest from the cable is known as the "tip", the middle connection is the "ring", and the (largest) connection closest to the wire is the "sleeve".