Potomac Consolidated Terminal Radar Approach Control

The Potomac Terminal Radar Approach Control (TRACON) controls approaches and departures in the airspace surrounding BWI, Andrews, Dulles, Reagan, Richmond and Martin State Airports
Here is the Official FAA website with VHF frequencies and graphics depicting sectors within the airspace controlled by the Potomac Consolidated TRACON (PCT).
All frequencies are in the AM mode. These are often used in parallel with one or more civil VHF aircraft frequencies, particularly during peak travel periods.
Flights leaving Martin State sometimes use Baltimore Washington International's frequencies
Flights leaving Andrews often check in on Reagan National's frequencies
Frequencies can be found here as part of the BWI information
The Tracon frequencies for Reagan National can be found here
The Tracon frequencies for Dulles can be found here

Member Airports/Air Bases