What is a 66 block?
A 66 block (also called a Type 66 block) is a punchdown block for terminating and cross-connecting twisted-pair telephone wiring. The "66" is a model type number from Western Electric, the Bell System's equipment maker, not a count of contacts. The block is a plastic base with rows of metal insulation displacement (IDC) clips; punching a wire into a clip slices through the insulation and grips the copper.
66 blocks were the standard for analog telephone (POTS) service and older business phone systems, and you'll still find them in wiring closets and near the demarc in older buildings.
How it's laid out
- Each row of clips handles one conductor. A common size takes a full 25-pair cable, one wire per row.
- On a split block, each row is divided into a left half and a right half. Incoming lines punch down on one side and station wiring on the other.
- Small metal bridging clips join the two halves. Pulling a bridging clip disconnects that line, which makes it easy to isolate and test a circuit.
66 block vs. 110 block
| 66 block | 110 block | |
|---|---|---|
| Main use | Analog voice, older low-speed data | Cat5e, Cat6 and higher data, plus modern voice |
| Performance | Not rated for gigabit Ethernet frequencies | Rated for high-speed data |
| Layout | Rows of exposed metal clips | Wires seated in plastic strips, with connecting blocks punched on top |
| Density | Lower | Higher; takes less wall space |
| Status | Legacy | Standard for data cabling |
The same 110-style contacts are used on the back of keystone jacks and data patch panels.
Punching down a 66 block
Use a punchdown tool fitted with a 66 blade; 66 and 110 blades are different, and many tools take interchangeable or reversible blades. Lay the wire in the clip with the cutting edge toward the waste end of the wire, then press until the tool snaps. The impact seats the wire and trims the excess. To identify a pair on a crowded block, use a toner probe; to check a phone line, clip a test handset (butt set) onto the terminals.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.2 lists punchdown blocks among the connector types, and objective 2.8 covers the punchdown tool. The objectives don't name 66 or 110 blocks specifically, but the split is worth knowing: 66 blocks are legacy telephone wiring, while 110 blocks are what you punch Cat5e and Cat6 data cabling onto.