A 66 block is a type of punchdown block used to connect sets of wires in a telephone system. They have been manufactured in three sizes, A, B, and M. A and B have six clips in each row while M has only 4. The A blocks have the rows spaced farther apart and have been obsolete for many years. The B style is used mainly in distribution panels where several destinations (often 1A2 key telephones) need to connect to the same source. The M blocks are often used to connect a single instrument to such a distribution block. 66 blocks are designed to terminate 22 through 26 AWG solid copper wire. The 66 series connecting block, introduced in the Bell System in 1962, was the first terminating device with insulation displacement connector technology. The term 66 block reflects its Western Electric model number.
The 25-pair standard non-split 66 block contains 50 rows; each row has four (M) or six (B) columns of clips that are electrically bonded. The 25-pair split 50 66 block is the industry standard for easy termination of voice cabling, and is a standard network termination by telephone companies—generally on commercial properties. Each row contains four (M) or six (B) clips, but the left-side clips are electrically isolated from the right-side clips. Smaller versions also exist with fewer rows for smaller-scale use, such as residential.
66 blocks are available pre-assembled with an RJ-21 female connector that accepts a quick connection to a 25-pair cable with a male end. These connections are typically made between the block and the customer premises equipment (CPE).