Area code 240 is an overlay for area code 301 in western and southern Maryland, including the suburbs of Washington, D.C., such as Rockville, Bethesda, Silver Spring, and Frederick. Introduced in 1997, it was the first overlay in Maryland and required the adoption of ten-digit dialing across the region. The 240 area code serves one of the most densely populated and economically important areas of the state, home to numerous federal agencies, biotech firms, and commuter communities. While 301 remains the legacy code, 240 is commonly assigned to mobile phones and new business lines. The 240/301 area is known for its affluence, diversity, and close proximity to the nation’s capital.