The demarcation point (DMARC) is the point at which the public switched telephone network (PSTN)ends and connects with the customer’s on-premises wiring (called Inside Wire or IW.) Don’t confuse this with a NIU, it’s the same but different.
For residential locations you might have started off with an old Western Electric Company (WECO) lightning protector:
Used around 1915. The phone company line from the pole connected at the top binding posts marked “L” for “Line”, the house wiring connected to “I” for “Inside” and G for Ground. In later years the Bell System added copper straps that bypassed the fuses… unsure why.
The Western Electric Type 58A Protector, circa 1900, protects against lightning and other high voltages.
After that they went to the carbon and porcelain blocks which were much smaller:
The carbon and porcelain block! In the presence of high voltage, the carbon blocks under the pressure of the spring would be moved to earth after the restraining glue melted. The whole thing had to be replaced.
In the 1980s, Ma Bell started using a gas-filled protector. In the presence of high voltage, the gas ionizes and provides a path to earth (ground). When the voltage is removed, the protector returns to its normal state. Not a one shot deal like the older carbon ones.
They got fancy with mounts & attached them to the side of your house:
Use before the advent of Network Interfaces.
Then we come to the modern Network Interface Device, Telephone Network Interface, NID, NI, dmarc:
NID with Telco access opened.
Check out http://catalog.corning.com/CableSystems/en-US/catalog/CategoryBrowser.aspx?cid=network_interface_devices_nids_web
What about businesses? Do they just have 66 of these on the wall, one for each line?
Don’t be crazy, they use a 66 block / M-Block / B-Block as a type of punchdown block to terminate the line.
Introduced in 1962, the term “66 block” simply reflects its Western Electric model number.
They could also have a 110 block:
110 style blocks allow a much higher density of terminations in a given space than older style termination blocks.