Where I live, any home that is around 100 years old will very likely have two front doors. Putting it simply, you open one door, step into a small space, and there will be another door in front of you. The space goes by many names, but they have a simple use; temperature regulation through draft prevention.
Sadly, they are often the first thing to go in renovations for open-concept designs.
Image Source: https://ca.pinterest.com/pin/457115430937940209/