Mattapan is a neighborhood in Boston. Historically a section of neighboring Dorchester, Mattapan became a part of Boston when Dorchester was annexed in 1870. As of the 2000 census, it had a population of 28,145. Like other neighborhoods of the late 19th and early 20th century, Mattapan developed, residentially and commercially, as the railroads and streetcars made downtown Boston increasingly accessible. Predominantly residential, Mattapan is a mix of public housing, small apartment buildings, single-family houses, and two and three family houses (known locally as "Three-Deckers"). Blue Hill Avenue and Mattapan Square, where Blue Hill Avenue, River Street, and Cummins Highway meet, are the commercial heart of the neighborhood, home to banks, law offices, restaurants, and retail shops. The new Mattapan Branch of the Boston Public library opened 2009, at a cost of more than $4 million. Mattapan is the original Native American name for the Dorchester area and meant "a good place to be," or "a good place to sit."