A 66 block is a punch-down block used to connect wiring for telephone systems and other low-speed low-voltage wiring applications. A punch-down tool is used to force solid wire into metal slots on the 66 block. The 25-pair standard non-split 66 Block encloses 50 rows; each row has four columns of clips that are electrically linked. Each row contains four clips, but the left two clips are electrically isolated from the right two clips. Blocks may be punched down manually, or purchased already wired at the factory.
The newer 110 block is replacing the 66 block, although 66 blocks still work well for traditional voice telephone applications.