Rollover, blocking and ghosting

These inter-related terms often come up in discussions of a keyboard's ability to register multiple keys at once. Often confusion exists over what exactly they correspond to, which is exacerbated by overzealous marketing departments using them in misleading ways, or using the terms interchangeably.

Ghosting

Ghosting is a potential issue where certain combinations of three or more keys can cause an additional key to be registered. Ghosting is a rare problem, and is a sign of a serious design flaw in the keyboard.

Many manufacturers misuse the term ghosting to refer to Blocking, or use the term 'anti-ghosting' to refer to a gaming optimized matrix.

Blocking

Blocking is when certain combinations of three or more keys result in one or more of the keys not being registered. Rather than being a design problem, blocking is deliberately implemented to prevent ghosting. The combinations of keys that block are dependent on the design of the keyboard.

Game Optimized Matrix

This term, and others like it, are used by manufacturers to describe a keyboard whose switching matrix has been designed in such a way to prevent blocking occuring on certain commonly used gaming combinations. It does not eliminate blocking, it merely tries to avoid it. In many instances, it is little more than marketing hype, and does not provide a tangible benefit over a regular keyboard. However, on some keyboards such as the Microsoft Sidewinder X4, the quality of optimiziation means that it is almost as good as a NKRO keyboard for most usage.

n-Key Rollover

n-Key Rollover, usually called NKRO, refers to a keyboard that suffers from neither blocking nor ghosting. In a true NKRO keyboard, any number and or combination of keys can be pressed simultaneously, and all will be registered correctly.

Interface-limited NKRO

Certain peripheral interfaces such as USB are not designed to support true NKRO. This, a USB NKRO keyboard will not suffer from blocking or ghosting, but is only capable of sending a limited number of keys over the interface at once. Additional keys pressed beyond the limit will generally cause some of the other keys to be dropped. USB is usually limited to 6 regular keys and 4 modifier keys. Some keyboards are able to cheat this limit, but sometimes at the expense of quirky behavior or incompatibility with certain operating systems.

2KRO, 6KRO and others

Sometimes, key rollover is described using a number. This practise is somewhat confusing because it means different things in different contexts. 2KRO is often used to describe a keyboard with blocking, and is meant to be read as "any combination of two keys are guaranteed to work, and some combinations of three or more keys will work, but others will not". 6KRO is often used to describe an interface-limited NKRO keyboard, and is meant to be read as "any combination of up to six keys are guaranteed, but no combinations of seven or more will work".

Some rollover quantities greater than 6, such as 12KRO, 18KRO or 22KRO are used to refer to heavily optimized matrices, or interface-limited NKRO keyboards that have got around the 6 key limit.

Transposition Errors

Transposition errors are when a number of keys entered in rapid succession are registered in the wrong order. For example, typing A then B then C, and the keyboard registering ACB or BAC. As with ghosting, this is the result of a design flaw in the controller, and is not considered acceptable failure. These errors have been spotted in certain low quality mechanical keyboards.