What is a milk door?

A milk door is a small hinged or slotted door built into the exterior wall of a house or other structure to allow the delivery of milk and other dairy products.

Milk doors were a common feature of houses in the early 1900s, when milkmen made daily deliveries of fresh milk to homes. The milkman would place the bottles of milk on a shelf inside the milk door, and the homeowner could then retrieve the milk without having to open the main door of the house.