While both function as punch-down blocks used in telephony to cross-connect electrical wires, 110 blocks are the modern and upgraded versions of the 66 block. The latter was designed for analog voice conversations, but the rise in usage of digital data communications shifted preference to the 110 block.
The 66 punch-down block is characterized by its ability to terminate 22 to 26 solid copper wire, and was introduced by the Bell System in the 1960s as a terminating device equipped with an insulation displacement connector feature. In general use, 66 blocks come with an RJ-21 female connector that is able to receive a male-end 25-pair cable.
Meanwhile, the 110 block was gradually entered into the foray in the 1980s upon the emergence of data communications, particularly computer processing. Category 5 jacks are equipped with 110 block terminals for wire connections. The older 66 blocks are not designed for Category 5 wiring systems.
Punch-down blocks provide a convenient method to connect wires without removing the wire's insulation or installing screws. These are largely used in key phone systems, most notably in PBX breakout boxes and 50-pin RJ-21 connectors. In order to punch the wires down into the slot, a device called the punch-down tool is used.