Telephone 66 block  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 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 pre-assembled with an RJ-21 female connector are available that accept a quick connection to a 25-pair cable with a male end. These connections are typically made between the block and the CPE