Door numbering is the systematic use of letters or numbers to identify individual entrances to a building. Signs with the designated letter or number are mounted on the exterior of the building above its corresponding door.

The signs should be of sufficient size and mounted a sufficient height above the door to be readable from several hundred feet away. If the door has a window above it, stickers commonly used for mailboxes can be placed on them, allowing for identification of the letter/number of the entrance from a close proximity. Signs can also be installed on the other side of the doors, inside of the building. The main entrance is usually designated "A" or "1" and the lettering or numbering is continued in a counter-clockwise fashion around the buildings. For reference, maps of the buildings are commonly labeled with the letters/numbers.
Another implementation of this is to place the stickers commonly used to number mailboxes on the windows of each classroom's emergency exit, allowing every classroom to be identified from outside the school.

Uses

One practical use of door numbering is for security

Security

Security is the degree of protection against danger, damage, loss, and crime. Security as a form of protection are structures and processes that provide or improve security as a condition. The Institute for Security and Open Methodologies in the OSSTMM 3 defines security as "a form of protection...

purposes, as well as things like helping direct visitors who are unfamiliar with the buildings.

Door numbering is commonly seen in schools, where it can be used to assist public safety officials in responding to incidents at the school. Door numbering is also beneficial for new students to find their classrooms on time.

Door numbering is also useful for telling drivers which exit to pick someone up at, directing deliveries coming to the school, as well as assisting in other administrative functions.

Door numbering is also a small sport, there are often competitions in small villages whereby people decorate their door numbers and are judged on style, artistic qualities and numerical merits.