Old Law Tenements are tenements built in New York City after the Tenement House Act of 1879 and before the New York State Tenement House Act ("New Law") of 1901. The 1879 law required that every habitable room have a window opening to plain air, a requirement that was met by including air shafts between adjacent buildings. Old Law Tenements are commonly called "dumbbell tenements" after the shape of the building footprint: the air shaft gives each tenement the narrow-waisted shape of a dumbbell, wide facing the street and backyard, narrowed in between to create the air corridor. They were built in great numbers to accommodate waves of immigrating Europeans. The side streets of Manhattan's Lower East Side are still lined with numerous dumbbell structures today.