A 66 block (also M-Block or B-Block) is a type of punchdown block used to connect sets of wires in a telephone system. 66 blocks are designed to terminate 22 through 26 AWG solid copper wire. The 66 series connecting block was the first IDC (Insulation Displacement Connector) type connecting block in the Bell System, introduced in 1962. The term "66 block" simply reflects its Western Electric model number.
The 25-pair standard non-split 66 Block contains 50 rows; each row has four 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 clips, but the left two clips are electrically isolated from the right two clips.
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 CPE (customer premise equipment).
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