They divide the kitchen or parlor from a back bedroom, letting a little light and air into the dark tunnel that was the typical 19th century slum apartment.
These windows have an appropriate name: tuberculosis windows. They were mandated by a 19th century city law requiring that tenements have cross ventilation to help reduce the spread of diseases like tuberculosis—the deadly “white plague” not uncommon in poor neighborhoods.
By 1901, however, the city passed the New Law Tenement Act, requiring exterior-facing windows in each room of new residences.
But just like bathtubs in the kitchen, some city apartments still have tenement windows—like this one on Avenue B.