A brownstone is a structure built with or faced with reddish-brown sandstone. The sandstone is itself called brownstone, giving rise to the buildings of the same name. Brownstone buildings are typically row house dwellings, though they may be freestanding buildings. Row houses are also known as terraced houses or townhouses. They sit side by side, sharing common walls, and provide medium density housing in urban areas. Brownstones are particularly associated with the cities of the Eastern United States. The Brooklyn area of New York City, for instance, has many brownstones, together called the Brownstone Belt. Boston also features many brownstone structures. Builders used brownstone commonly in the 19th century, taking rock from the brownstone quarries of the Northeast. Because brownstone is a soft, sedimentary stone, it doesn't weather as well as other building materials, and this made it lose favor as a building material. Brownstone's color comes from oxidized iron.