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Mechanical Keyboard Terms Definitions, explanations, and examples of commonly used mechanical keyboard terms.

While 6KRO and NKRO are by far the most common levels of mechanical keyboard 'key rollover' functionality, there are still other numbers used. The number represents how many keys you can press at once and still have them be recognized by the keyboard. Most membrane keyboards are 2KRO and 3KRO. Some mechanical keyboards which aren't quite NKRO come in 20KRO flavors.
Currently, the most commonly seen 'Key Rollover' functionality of USB interfaced keyboards. 6KRO means you can push up to 6 keys + modifier keys (CTRL, ALT, etc.) at once and the keyboard will recognize all of your key presses. 6 keys is usually more than enough since most aren't usually mashing 7+ keys simultaneously, even with video games. However, if you do utilize 7+ keys at once, you can always get a keyboard with NKRO (no key rollover).
Actuation Force
Activation Point (or Operating Position) is the key travel distance where the key is actually recognized by the keyboard. Actuation force is the force required at this point. Put simply, it's how hard you have to press the key for it to be recognized.
Bottoming Out
Bottoming out is pushing a switch all the way down.
See "Key Bounce"
Click vs Clack
'Click' is the noise made by the switch when activated and 'Clack' is the noise made when it bottoms out.
Clicky switches make an audible 'click' when typing. Clicky switches are typically preferred by typists, but are noticeably louder than other mechanical switches. Use with caution near noise sensitive co-workers or spouses :)
Compact Layout
Some keyboards have approximately the same number of keys as a fullsize keyboard but are laid out in a different way to reduce width.
Double Tap
Hitting the same key in rapid succession. See also 'Triple Tap'. Most commonly used in video games.
Ghosting is when a unintended key press is sent (a "Ghosted" character). This is normally handled in firmware by limiting the number of simultaneous key presses. "Anti-Ghosting" is sometimes used by keyboard manufacturers however to mean N-Key Rollover (NKRO). Presumably because it sounds cool.
Happy Hacking Keyboard sold by PFU systems and using Topre switch technology.
Key Bounce
Key Bounce (also known as 'chattering') occurs in mechanical switches and can cause one switch press to be detected as multiple presses. Typically this is filtered out; however, a defective switch can bypass these filters and output multiple signals for a single key press.

A mechanical switch, being mechanical and generally filled with springs and thin metal, will tend to bounce around several times making and breaking electrical contact before coming to rest. This often ranges up to 5ms. The firmware designer of the keyboard controller takes this into account and sets the scanning rate high enough that keys are being reported accurately but low enough that multiple erroneous key presses aren't sent to the PC.

This not to be confused with the 'key speed features' that some Gaming Keyboards utilize. An example is a Ducky DK9008 with the 1x, 2x, 3x, and 4x speed features controlled via the keys at the top right of the board (typically used for FPS style video games). These speeds dictate how many times the signal will be sent in a given time frame. For example, at 4x speed, pressing the 'S' key once will typically look like 'SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS' in notepad.
Key Lifecycle
The lifespan of a key switch, or how many times a key switch can be activated before failing. Usually represented by 20 - 50 million key strokes for mechanical keyboards.
Key Rollover
Typically denoted by #KRO with 'N' representing unlimited key rollover; Key Rollover is the maximum number of keys you can press at once and still be recognized by the keyboard.
Keyboard Matrix
Since connecting them all individually would require an awful lot of lines that would be a mess to route and scan, the key switches are laid out in a matrix. A glimpse of one is shown in the rubber dome pic above. How the switches are laid out determines the NKey characteristics of the keyboard (switches on the same row behave better than if you try and hold keys on different columns).
Linear switches have no bump before bottoming out. They travel smoothly and increase in resistance as you reach the end of the key press.
Mechanical Keyboards
In contrast with the membrane keyboard, the keys on a mechanical keyboard have individual keyswitch mechanisms that register keystrokes instead of sharing a membrane sheath with all other keys. This results in a different feel when gaming or typing. For instance, Blue Cherry MX Switches are a form of clicky and tactile mechanical keyswitches that deliver a very distinct tap to the user's fingers when a key is actuated accompanied by an audible click, quite similar to the experience of performing a mouse click. As a result, the tactile feedback experienced by the user on a mechanical keyboard is significantly different from a membrane keyboard, with some users demonstrating a strong personal preference for this type of keyboard feel.
Membrane keyboards
In membrane keyboards, the keycaps are positioned above rubber domes which actuate the keystrokes through a plastic sheath. This plastic sheath spreads out over the entire keyboard under the keycaps and keystroke actuation occurs when the key is fully depressed. The travel distance of the keyboard is determined by the thickness of the layers of the sheath and is usually 3.5 to 4 mm. The membrane-type keyboard is by far the most common type of keyboard architecture used today.
NKRO is the 'Unlimited' version of Key Rollover. This means that each key is scanned completely independently by the keyboard hardware, so that each key press is correctly detected regardless of how many other keys are being pressed or held down at the time. Originally only available through PS/2 interfaces, brands such as Ducky have created keyboards that can offer NKRO through both the PS/2 and USB interface.
Normally Open
A term used to describe electrical contacts that are open in the rest position.
In the world of mechanical keyboards "Otaku", refers to blank key caps (caps with no letters).

Outside of keyboards in Japan, "Otaku" refers to young people who are highly skilled in or obsessed with computer technology to the detriment of their social skills.

There is some slight correlation between the two as it does take a high amount of technical skill to touch type on letter-less key caps.
A distinct noise you can sometimes hear from the spring inside a mechanical switch.
Plate Mounted vs PCB Mounted
Switches can be mounted onto a metal plate over a PCB (circuit board) or directly to the PCB. This tends to affect switch feel and build quality of the board.
Polling Rate
While it is very useful for mice, it's just about meaningless for keyboards. Let's assume for a minute that all switches have the 5ms debouncing time of Cherry MX switches (which is being very generous). Even if you had super human speed and reflexes, every single key would be delayed by at least that much. So really, any polling rate over 200Hz (at best) is absolutely useless, and nothing but market hype. It may even be a bit detrimental, because you'd be wasting CPU time polling the keyboard unneededly. And unlike USB keyboards, PS/2 boards aren't polled at all. They simply send the signal to the PC whenever they are ready to, which causes a hardware interrupt, forcing the CPU to register that keystroke.
PS/2 Interface
The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. Even though it's older technology than USB, from a performance standpoint, PS/2 is a superior connector type.

However, it is important to note that the performance gains over USB are virtually irrelevant in a modern computer. The bottom line: use PS/2 if it's available, but don't write-off a keyboard you really like if it's USB only.
Response Times
See 'Polling Rate'
Riding The Activation Point
This is different than "Double Tap". It's a useful for technique for "spamming" one key stroke by keeping the switch in the middle of it's travel distance so you can send multiple keypresses without fully releasing OR fully depressing the key. You can't really do that with a rubber dome.
Rubber Dome Switches
Rubber dome switches, most commonly referred to as polydomes, are formed polyester domes where the inside bubble is coated in graphite. While polydomes are typically cheaper than metal domes, they lack the crisp snap of the metal domes, and usually have a lower life specification. Polydomes are considered very quiet, but purists tend to find them "mushy" because the collapsing dome does not provide as much positive response as metal domes. For either metal or polydomes, when a key is pressed, it collapses the dome, which connects the two circuit traces and completes the connection to enter the character.
Sprue / Keycap Sprues
In casting, a sprue is the passage through which a molten material is introduced into a mold. The term also refers to the excess material which solidifies in the sprue passage.

Since the vast majority of key caps are plastic mold injected, you will notice sprues on the back of nearly all keycaps. Sprues are only visible from the back of the keycap and are sometimes mistaken for a manufacturer's defect - as a hole or extra plastic; however, sprues are an unavoidable byproduct of the manufacturing process. Unless you type with a backlit keyboard upside down, odds are you would never notice them.
Sometimes referred to as 'tactile bump'. Tactile switches have a (sometimes subtle) bump you can feel in the key press before bottoming out. This bump typically represents the actuation point.
Tactile point / Tactile force
Similar to the Activation point/Activation force point but at the top of the tactile "bump"
Tenkeyless (TKL)
Term for keyboards that don't have a numeric keypad section and are therefore narrower and some say more ergonomic. Some have embedded number pads in the right portion of the main key cluster like some laptops. Tenkeyless keyboards are often abbreviated TKL.
Travel Distance
This refers to the actual distance the keys of a keyboard have to be pushed down before the keystroke is recognized. Laptops and notebook computers, in general, have much shorter travel distances on their keyboards compared to standard desktop keyboards. Some gamers prefer keys with a shorter travel distance as they feel that it requires less effort to actuate the keys, however, others prefer a large travel distance as it makes keystrokes more distinct.
Type A Defect
A 'Type A Defect' is a defect MK is made aware of by the manufacturer before we order the inventory. MK agrees to a discounted price and passes that discount along to our customers. Defects of any type are always conspicuously listed in the product's title and description.
Type B Defect
A 'Type B Defect' is a defect MK is made aware of only after we receive the inventory. MK makes every effort to address these Defects with manufacturers - typically by returning the defective merchandise. In the event the manufacturer refuses to allow the product's return, MK will sell the merchandise at a hugely discounted price - often below our cost. Defects of any type are always conspicuously listed in the product's title and description.
Universal Serial Bus (USB) is an industry standard developed to standardize the connection of computer peripherals, such as keyboards. Most modern computers offer a minimum of 6 USB ports and some keyboards offer USB pass-through (USB ports built into the keyboard) which allow users to plug other USB devices directly into their keyboard.

While PS/2 may have a slight edge in performance, features like USB pass-through often give USB connected keyboards an edge in ancillary functionality.