the phone company is punched down on the left side of a split-type block in pairs. The right hand side of the block is wired to the customer premises equipment with jumper wires. Bridging clips are used to connect the two center terminals, connecting the left-hand side of a split block with its right-hand side, thus completing the circuit. The clips form the point of interface between the subscriber and the provider. The bridging clips can be easily removed by either the subscriber or phone company personnel for trouble isolation, allowing the ability to split a circuit and determine in which direction trouble may exist. An orange insulating cover attached to a 66 block denotes its designation as a demarcation point by the local exchange carrier. Modern 110 blocks (Originally designated as 88 blocks) have largely supplanted 66 blocks for new commercial installations at the end of the 20th century, as the capability for a circuit to carry digital data overlaid its ability to carry analog voice conversations. 110 block termination is almost always compliant with Category 5 (or higher) and therefore capable of supporting 100 MHz (or faster) signaling. Compared to 110 and higher-density wire terminating blocks, 66 blocks are physically large; and because of their maximum 16 MHz Category 3 signaling compatibility, they are ill-suited for high speed (faster than 10BASE-T) data circuits. However, special Category 5e Certified 66 blocks are available from manufacturers such as Siemon which meet all standards for Cat5e termination. Split 50 66 blocks are still used as network interface blocks in distribution frames to interconnect circuits with bridging clips, but are primarily limited to narrowband circuits such as POTS/DSL, DS0, or DS1 circuits. See also National Electric Code 25-pair color code 110 block References Telephony equipment Local loop