Phone numbers are seven digits long with a three-digit area code at the beginning. As the population of the United States grows, new area codes get added to increase the amount of available phone numbers. Therefore, area codes serve areas based on their population density. Some states have only one or two area codes, while others have many more. For instance, as of 2017, Maryland has five area codes. It’s a small state based on geographic area, but it has a high population density that requires more area codes. The following infographic explains how the area codes in Maryland are set up.

There are five area codes in Maryland — two main codes and three overlay codes. They are 301, 410, 240, 443, and 667. Area code 301 was first, established in 1947. Now, 301 covers only the western part of Maryland. Area code 410 was added in 1991, 240 and 443 were added in 1997, and 667 was added in 2012. Area code 410 covers eastern Maryland while 240, 443, and 667 are all overlay codes that service both 301 and 410.

Originally, people were opposed to adding overlay area codes to Maryland because nobody wanted to dial 10 numbers instead of seven. However, it was necessary because the population density grew so quickly. The following infographic shows which cities are serviced by the two main area codes and how the overlay codes work. You’ll also see fun facts and a brief history of how the area codes were introduced to the region.