For much of Philadelphia's history, the typical home has been the row house. The row house was introduced to the United States via Philadelphia in the early 19th century and, for a time, row houses built elsewhere in the United States were known as "Philadelphia rows". A variety of row houses are found throughout the city, from Victorian-style homes in North Philadelphia to twin row houses in West Philadelphia. While newer homes are scattered throughout the city, much of the housing is from the early 20th century or older. The great age of the homes has created numerous problems, including blight and vacant lots in many parts of the city, while other neighborhoods such as Society Hill, which has the largest concentration of 18th-century architecture in the United States, have been rehabilitated and gentrified.