The Double Key: The Key That Goes Both Ways

Written by Jens Sethmann

Translated from German by Abha Vardhan (2019)


The Double key, also known as the forced locking key, close constraint key, and Berlin Key, is a double-sided key with a unique locking system. It is also referred to as Durchsteckschlüssel in German, which means push-through key. It is designed in a way that forces people to close and lock their doors, usually the main door of the apartment so that the same key is shared by many people.

Anyone who sees this front door key for the first time will be surprised by its shape as there is an identical beard at both ends with no key head.

These peculiar keys have been used in tenant buildings in Germany for over 100 years now and are astonishing. They are still used today in some places due to the ingenious locking system. They were quite popular until the 1970s and found on many Berlin key rings, but now they are replaced by modern locking systems. Older people in Berlin must be aware of the double key as it became very popular during those times.

What Makes The Double Key Unique?

As a rule, one key blade is enough to close a lock. The Berlin Key is unique because it has two key blades (the portion which starts the bolt); one at each end of the key, rather than the usual single blade that you normally find on a standard key. The Berlin or Double Key has a brass carrying cap. These are unique keys with the name of the original company name (Albert Kerfin & Co) written on them.

In those times, the handles of front doors were simple and could be easily tampered with compared to today’s highly secure and contemporary locks. The main feature of the double key was an efficient mechanism that would not let the user pull out the key unless they lock it from the other side of the door. The keyhole comes with two passages 45 degrees to each other.

How The Double Key Works

Here’s how the double key works:

  • The owner of the house locks the door in the evening. From this point, it was made sure that each person going through the door will keep it closed.

If you want to unlock the door, you need to insert the key in the lock ordinarily, turn it one and a half times, and push it through the other side. It will unlock the door.

If you try to pull out the key again at this point, you won’t be able to. If the door is open, the key remains in the keyhole of the lock. Whatever force you apply, use any sort of plier or tools, or do anything to take out the key, you won’t be able to take out the key.

  • Once you have unlocked it, you must push it all the way through the keyhole of the lock so that it becomes accessible from the other side of the door. When you open the door, move in and closed the door, you will need to lock it again then pull the key from the lock.

It is worth noting that you cannot take out the key from the lock when the door is open. In addition, it is not possible to lock the open door with this ingenious lock system.

These double keys are exclusively created to lock and unlock the front door of the house or the gate that opens into a common block.

Who Invented the Double Key?

The idea of the double key is as wonderful as it is simple. The inventor of these exclusive double keys was locksmith Johann Schweiger from Berlin.  Approximately 103 years ago, the 24-year-old lad put his great idea into practice and later applied for its patents, the mortise lock and key in 1912. They are still referred to as the "System Schweiger" today.

Johann Schweiger wanted to ensure safety for his associates who usually lived in tenements that were linked through courtyards, which made them easily accessible for intruders. He came up with the brilliant idea of the double key before the first World War in order to enhance the security of Berlin tenants. In Berlin, tenants used to live in big buildings with about 20 tenants and two courtyards and it was quite uncomfortable for residents since everyone could come into the courtyard.

The Double key he invented made sure that the tenant would close the door and in turn ensure their safety. This double key is still used in several old parts of Germany today,  where houses are connected through backyards.

The main idea behind creating this type of key was to eliminate the job of caretakers who usually had the task of opening the doors for the tenants throughout the night. This is because, at that time, big apartments were created from the central part of the city. The use of the double key saved on having a caretaker watching for undesirables. Nowadays, it has been replaced by modern electronic keypads.

The property owner introduced the double key in order to make it more convenient for both the caretaker and the tenants. It made it easier for them to control the entry mechanism of the building. When using the key correctly, the tenant can unlock the door to the home without the assistance of the caretaker.

The Kerfin company experienced a lot of success with this locking system in the 1950s as many housing cooperatives used them for better security and convenience. More than half of the population of Berlin had a double key in their pocket, especially in districts such as Wedding and Kreuzberg.

It has been estimated that more than 8000 to 9000 old buildings are still using Mortise locks with double keys today. The key business is still run by the family and the tradition will continue even if the business is not flourishing so well today because of modern key locks taking over.

However, the double key still remains the trademark of the company and graces the company logo.

The Symbol of the City

The "Schweiger" mortise lock was installed on so many Berlin front doors that the extraordinary key became the symbol of the city. The Kerfin Company is still run by the Schweiger family today and has also kept Wedding as the company's headquarters. They have created a Berlin "Buddy bear" as a typical sign on the double-bearded key.

You will sometimes find outdated original keys in junk boxes on the flea markets with "Kerfin & Co. Berlin 65" engraved on the bridge. Souvenir hunters have already discovered them as prey.

The Essay on Double Keys by Bruno Latour

Surprisingly, the double key has attracted the attention of scientists too. One of the scientists who worked for the WZB Berlin Social Science Center, Bruno Latour, wrote a great article: The Berlin key or how to do words with things, on the efficiency and usefulness of the double key.

This essay was first published as La clef de Berlin et autres lecons d'un amateur de sciences, La Dècouverte in 1993. Later, it was printed as the first chapter in P.M. Graves-Brown's Matter, Materiality and Modern Culture.

This article was about 15 pages long in which the writer explained about the Berlin Key in detail. He explained that the Berlin Key is created in a way that after unlocking the door from the inside, you can recover the key when you have closed the door upon yourself. This mechanism prevents leaving the door open for a long time and makes it easier to stop intruders getting in.

Bruno Latour wrote that a purposefully designed object may offer improved functionalities and a large range of useful prospects. He goes on to explain that the Berlin Key is a way to remind people of the significance of locking their doors.

Latour was really fascinated with this invention and the functioning of the double key, and he concluded that the key symbolically represents opposites, such as residents and thieves and inside and outside.

The Double Key for Better Safety and Security from Intruders

The Double Key will not let the owner inside the house until they lock the door first. Although simple, the idea was highly effective. It worked to ensure security in houses with several tenants, many courtyards and no caretaker. The use of the double key made sure that the main front door remained locked at night. 

Although the double key is not much in trend these days, it was definitely a remarkable invention by the locksmith Johann Schweiger from Berlin. His idea was novel, fresh and functional which even intrigued the scientist, Bruno Latour who later wrote a long essay on its functioning and usage.