A vestibule is a small entrance room or hallway that connects the outside of a building to the main interior spaces. When you walk into many schools, churches, or apartment buildings, you first step into a vestibule before reaching the main hallway or lobby. It's that in-between space where you might shake off snow from your boots, hang up your coat, or wait for someone to buzz you in.

Vestibules serve practical purposes. They act as a buffer zone, keeping cold air, wind, and rain from blasting directly into the main building every time someone opens the door. In winter, a vestibule traps that first rush of freezing air so the people inside stay warm. Many vestibules have two sets of doors (one leading outside, one leading inside) to make this barrier even more effective.

Today, you'll find vestibules in train cars too, where they're the small platforms between cars where passengers can stand while doors open and close. Think of a vestibule as architecture's way of saying, “Take a breath here before you enter.”