The Chesapeake and Potomac Telephone Company, Old Main Building is a historic structure located in Downtown Washington, D.C.

As the number of businesses in Washington increasingly relied on telephone service, Chesapeake and Potomac Telephone Company built this building as its new main exchange.

Designed by architect Leon Eidlitz, it was the first of a complex of buildings the company would construct at this site.

The building houses what was considered to be the largest telephone switchboard at the time.

[2] The new system eliminated the multiple rings on party lines and the need for an operator to interrupt the line to determine whether a subscriber had completed a call or wanted to receive calls.