I'm looking for a word to describe a built-in device that prevents the user from incorrectly using/operating the thing it is on.

e.g. A tilt and turn window has a device which prevents the user from turning the handle further when the sash is open which could make it drop out and fall on the person.

It removes the possibility/opportunity for a user to do something wrong that would cause the window/object to fail.

I've found

  • "Faulty operation lock"
  • "anti-mishandling device"

I find them unclear, sound unnatural and only appear on the websites of foreign companies (hence bad translations).

"Fail safe device" is ok, but I find it too vague in this context.

Any ideas?

  • 1
    OP, if you literally want the term for that thing IN A WINDOW, you should ask that in the title. it is a totally different question from the "general question".
    – Fattie
    Commented Nov 20, 2017 at 14:26
  • just fyi a failsafe is different than an interlock. An interlock prevents improper operation, a failsafe is designed so that failures are benign. In your case, the window has not failed, it is just designed so that the window removal mode is not inadvertently entered.
    – Jim
    Commented Nov 20, 2017 at 14:54

12 Answers 12


The standard term used in all types of industry for this is an interlock.

Interlocks can be implemented with software, electrical circuitry, or by mechanical means.

An example of a software interlock is a piece of code which checks a number of variables before allowing a certain action to execute.

An electrical interlock is usually a chain of sensors, switches, or relays that have to be set in the correct state to allow an operation to take place. A common example is the sensors in elevator doors which prevent them from crushing a person when they sense someone is obstructing the door's automatic closing, or to prevent the doors from opening while the elevator is in motion or stopped between floors.

Mechanical interlocks are physical devices that prevent a mechanical system from entering an unwanted state. Locking bearings and corresponding grooves in the gear selector shafts of a manual transmission, for example, prevent more than one gear from being selected at a given time.

The linked wikipedia article is not entirely accurate - it suggests the term is more specific than it actually is in practice. For example, it says :

For example, in a typical household microwave oven, the switch that disables the magnetron if the door is opened is not an interlock.

While I think in practice most engineers I know would certainly refer to such devices as interlocks, formally. The article's talk section has some good citations on the erroneous nature of that particular statement.

  • 1
    Awesome. Interlock does actually seem to be the perfect answer. Since it does not seem commonly used in windows, I'm going to stick with "Integral fail safe device" for now. I need something short that most everyone (e.g. regular people) will immediately understand. If you're interested, the exact thing I mean is pictured here: fensterhai.de/fileadmin/user_upload/Montageanleitung/…
    – SonOfPingu
    Commented Nov 17, 2017 at 15:22
  • @J... my experience with laser systems is that the same arrangement used in microwaves is always referred to as an interlock
    – Chris H
    Commented Nov 17, 2017 at 16:39
  • @ChrisH Yep - anything Class IV will have a big, bold connector at the back labelled INTERLOCK.
    – J...
    Commented Nov 17, 2017 at 16:57
  • @CMNEN - What does that thing do? Keeps you from being able to lock the door? I googled Fehlbedienungssperre and that's the video I got. Translated, it's "tampering device" (... tamper proof ?). But I'd assume it's a lockout - for a lock - which is not the easiest thing to google.
    – Mazura
    Commented Nov 17, 2017 at 22:41
  • 1
    For a canonical (and tragic) example of what happens when you omit a necessary interlock, search for Therac-25.
    – Ex Umbris
    Commented Nov 19, 2017 at 8:21

The term used by the UK Government Health and Safety Executive is 'safeguards'.

[Under 'What do I have to do ?'. . . Before you start.]

Check that the machine is complete, with all safeguards fitted, and free from defects. The term ‘safeguarding’ includes guards, interlocks, two-hand controls, light guards, pressure-sensitive mats etc. By law, the supplier must provide the right safeguards and inform buyers of any risks ('residual risks') that users need to be aware of and manage because they could not be designed out


  • 1
    +1 The other answers might be the more technical term, but if you're looking for a more everyday term, this does nicely. As an American, this was the first word than came to my mind as well, so I'm certain that this isn't British specific.
    – jpmc26
    Commented Nov 19, 2017 at 7:37
  • As an everyday term this is fine, but a safeguard is a much broader term that includes all types of things that aren't interlocks (ie : that aren't things which prevent unwanted operation). As example, things like shields, guards, and deflectors would be included - items that prevent fingers, hands, etc, from entering dangerous parts of the equipment (or to prevent work or swarf leaving the equipment at speed). In these cases such items are needed precisely because the system is not interlocked and yet still relies on these physical barriers being installed for safety.
    – J...
    Commented Nov 19, 2017 at 9:53

There is an enormous variety of such devices, so the terminology varies depending on the particular usage, but here are some blanket terms:


A mechanical device or system architecture which requires some external set of requirements to be met in order to continue operation. In the example you gave, the device which prevents the handle from turning with the sash open is the interlock. In this case, it is a safety interlock.

Source: I'm an aerospace engineer

A safety interlock can be a:

Behavior-shaping constraint

These constraints force the user to make the device safe, often by way of an interlock. For example, in manual transmissions, there is an interlock which requires the driver to depress the clutch petal before starting the vehicle. This system is a behavior-shaping constraint, since the driver soon learns the process, and instinctively makes the system safe.



Borrowed from Japanese, poka-yoke means "mistake-proofing" or "inadvertent error prevention". Specifically, it is a design wherein a system will bring the operator's attention to improper conditions. Thus, a poka-yoke doesn't actually prevent misuse like an interlock does, but instead relies on the operator to correct the condition.

For example: when keys are left in an automobile ignition, modern vehicles will make a persistent dinging sound when the driver door is open in an attempt to alert the driver, who might otherwise lock the keys in the vehicle.

This is in contrast to an interlock, which would instead prevent the doors from locking while the key is in the ignition.



This is the the general design principle which attempts to predict ways a device could be misused, and designing in a way which makes such misuse difficult or impossible. https://en.wikipedia.org/wiki/Defensive_design

  • 1
    +1 for interlock (or interlocking)
    – Muzer
    Commented Nov 17, 2017 at 16:08
  • A slang term for a specific kind of error-prevention device is molly guard, which is probably easier explained by some photos than a text explanation. (It's a shield to guard the button from Molly, who is liable to press it.)
    – user73481
    Commented Nov 17, 2017 at 17:36
  • 1
    Poka-yoke is the first thing I thought of as well.
    – delliottg
    Commented Nov 18, 2017 at 4:03

I agree with many of the other answers here in regards to a generic term, but there is a term that relates specifically to your examples that I have not seen.


A "stop" is a piece on a mechanical device that prevents it from moving too far. The window handle would have a stop that prevents you from turning it too far, and the hinge of the window itself would have a stop that prevents it from opening too far. This is also used for a "door stop" that keeps the door from hitting a wall.

  • 4
    With the context given, this is the correct answer.
    – Mazura
    Commented Nov 17, 2017 at 22:08
  • 1
    This is the term that I would understand, as someone whose only expertise with windows and doors is owning and operating them.
    – 1006a
    Commented Nov 18, 2017 at 16:01
  • Yes, it's simply a stop or a "dog".
    – Fattie
    Commented Nov 20, 2017 at 14:23
  • @Fattie, A dog ( en.wikipedia.org/wiki/Dog_(engineering) ) is a part that locks two parts together to synchronize movement or to stop movement. A stop restricts from moving too far, but does not hold something in place.
    – JPhi1618
    Commented Nov 20, 2017 at 14:30
  • hey @JPhi1618 - hmm, on say a ship (picture one of those arctic fishing boats that feature in TV documentaries these days!) .. imagine you have, say, something that slides along a bar. a "dog" is nothing more than a projection of metal (like .. a dog, a penis) that the sliding thing runs in to and is thus stopped. it's just a "stop", a stick of metal that sticks out. (I've often wondered if a dog (ie, the opposite of a bitch) is so called because it has a dog, or if dogs are named after dogs!) Anyway - you're totally correct, in engines and stuff a "dog" can kind of mean "gripper" - but -
    – Fattie
    Commented Nov 20, 2017 at 14:47

Similar to Adrian's safety feature is safety catch.

In British English at least, I'd be more likely to refer to the specific button or switch on a device as "the safety catch" instead of the "the safety feature". In the US, in relation to guns, I believe this is often just shortened to "the safety".

  • Thanks. in AE it is definitely for firearms primarily. Thanks for the input, I didn't know there was any other meaning of it. However, the device I mean is integral, it's not a button or switch the user even sees, much less operates.
    – SonOfPingu
    Commented Nov 17, 2017 at 12:06
  • @CMNEN - Then it's just an integral safety (device). Most equipment in your home comes equipped with, or is protected by, multiple safeties (North American: short for safety lock – Google). E.g., the T&P valve on your HW tank, or the limit switches and the fuse in your furnace, and every other CB or fuse in your house.
    – Mazura
    Commented Nov 17, 2017 at 22:04
  • I think that's a bit different - it's something you actively turn on for safety. I believe the OP is asking about a safeguard, a stop.
    – Fattie
    Commented Nov 20, 2017 at 14:23
  • I may be wrong about that - maybe that's exactly what you call the stop on a window that stops it going out to far .. the safety stop .. "safety latch" maybe??
    – Fattie
    Commented Nov 20, 2017 at 14:29

Although it sounds a bit pejorative, one example is fool-proof.

In the early days of computer design, peripherals had to be plugged into a PC using specific outlets. Mouse, keyboard, printer, etc each had ts own outlet. If the user plugged any device into the wrong place, damage could occur.

At first some manufacturers color-coded the cords, but this did not prevent user errors. Finally they changed the shapes of the plugs; making it impossible for the user to miss-configure the setup.

They fool-proofed the design.

  • Thanks. "Fool proof" is the right direction and a great example, but it doesn't quite capture the German idea I'm trying to explain succinctly.
    – SonOfPingu
    Commented Nov 17, 2017 at 12:09

The interlock answer is a good one. It has connotations of a connection between two systems, to stop them both being in conflicting states. It is also a fairly technical word for a lay audience.

A more passive device might be called a guard, which Google defines for me as "a device worn or fitted to prevent injury or damage."

Blade guards, sneeze guards, molly guards, finger guards - these are all "devices that prevents the user from incorrectly using/operating the thing it is on." without being interlocks.


I can't think of a single word but safety feature could be used to describe the device in your tilt and turn example.

  • This is perfectly correct.
    – Fattie
    Commented Nov 20, 2017 at 14:24

I'm going with "Integral fail-safe device" which is still somewhat vague but comes closest to the idea without being a full sentence.

  • 1
    Fail safe means the device ends up in a safe state when it fails, e.g. a fuse that cuts power if it overloads, or a nuclear reactor that drops control rods into the fissile material to prevent meltdown if coolant stops flowing. They do not limit the capacity of a user to do something they shouldn't.
    – beldaz
    Commented Nov 19, 2017 at 19:32

I see no one has mentioned the informal term idiot-proof yet. This is to be distinguished from fool-proof, a term that does not apply here.

A fool-proof system cannot be cheated (fooled) by someone trying to cheat it.

An idiot-proof system on the other hand, is designed to be robust to unintentional errors committed by an intended user.

  • In tech, I have heard the term 'marketing-proof'... :P
    – smci
    Commented Nov 18, 2017 at 20:17
  • I believe, the OP is asking for the term for the "thing" that is indeed there to idiot-proof the device.
    – Fattie
    Commented Nov 20, 2017 at 14:24

What about "Precaution System or Unit" ?

  • Thanks, but that doesn't really sound like English nor is the meaning clear.
    – SonOfPingu
    Commented Feb 12, 2018 at 8:49

I immediately think of:

governor (n.)

a : an attachment to a machine (such as a gasoline engine) for automatic control or limitation of speed

b : a device giving automatic control (as of pressure or temperature)

Mechanical sense of "self-acting regulator" is from 1819. (Etymonline)

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