In the City of Boston, the Twelfth Suffolk District includes Precincts 8, 11 and 12 of Ward 16, Precincts 4, 11, 12, 13 and 14 of Ward 17, and Precincts 1, 2, 4, 5, 6 and 21 of Ward 18, primarily covering sections of Dorchester and Mattapan.

Dorchester is Boston's largest neighborhood and also its oldest, founded a few months before the city itself. The neighborhood's
 image of Dorchester and Mattapan signs
historical diversity is exhibited in its architecture, from the old Victorian homes of wealthy Bostonians to the multi-family dwellings of later groups of immigrants. Today, Dorchester retains its diversity. Its main thoroughfare, Dorchester Avenue, connects many close-knit neighborhoods and thriving commercial districts of all kinds. Dorchester is also home to the University of Massachusetts at Boston and the John F. Kennedy Library.

Mattapan was annexed to Boston in 1870 as part of Dorchester. As with other neighborhoods of the time, Mattapan developed as the railroads and streetcars made downtown Boston accessible. Predominately residential, Mattapan is home to a mix of single homes as well as two and three family houses. Mattapan Square, where Blue Hill Avenue, River Street, and Cummins Highway meet, is the commercial heart of the neighborhood.