Until shortly after the Civil War the well to do in New York City lived in private houses, and only the working class and the poor lived in multiple dwellings. By 1870 “French flats” had been introduced as a new concept in middle class living. They were distinguished from the tenement houses of the poor and working class by amenities such as parlors, separate dining rooms, a small servant’s room, and indoor plumbing. The apartment concept proved to be attractive and by 1900 half of the middle class were living in multiple dwelling units. Several factors facilitated the move to apartment living. Changes in the law in 1901 allowed buildings to rise to heights twice the width of the street, resulting in buildings of ten to twelve stories (especially along broad avenues). Also pivotal was the development of mass transit up Manhattan’s west side when the IRT Broadway line subway opened in 1904. The Upper West Side experienced a boom in the erection of heavily ornamented grand apartment buildings in various architectural styles, but most notably Beaux-Arts. These new buildings offered apartments of nine to twelve rooms and many duplexes with large rooms with high ceilings, lavish interior details, as well as well-appointed bathrooms and kitchens, and large closets. Apartment buildings provided luxuries and conveniences not possible in most private dwellings. By 1929 almost all upper and middle class residents in Manhattan were living in apartments.