Rollover is the ability of a computer keyboard to correctly handle several simultaneous keystrokes.
Normal typing 
During normal typing on a conventional computer keyboard, only one key is typically being pressed by the user at any given time; each key is released before the next key is struck. However, this is not always the case. When using modifier keys such as Shift or Control, the user will intentionally hold down the modifier key(s) while striking and releasing another key. Rapid typists may sometimes inadvertently press a key before releasing the previous one. Certain unusual forms of keyboarding require multiple keys to be struck or held down simultaneously- for example, Braille2000 keying requires that as many as six keys be struck at once. Some computer games have interfaces requiring holding down keys (other than the usual modifier keys) while pressing and releasing other keys.
n-key rollover 
Certain high-end keyboards have "n-key rollover". This means that each key is scanned completely independently by the keyboard hardware, so that each keypress is correctly detected regardless of how many other keys are being pressed or held down at the time.
However, to reduce cost and design complexity, most keyboards do not isolate all keys in this way. Instead, they use a matrix of key switches, without any isolation diodes, that assumes that only a limited number of keys will be held down at any given time.
Multi-key rollover 
A keyboard with "2-key rollover" can reliably detect only any two keys used simultaneously; in other words, a user can hold down any key on the keyboard and press a second key, and be sure that the keypress is correctly detected by the computer. However, if the user has two keys depressed and attempts to strike a third key, the third keypress may create a "phantom key" by shorting out the switch matrix. This is not acceptable for quality keyboards because there are many cases when more than two keys need to be depressed at the same time, such as Ctrl-Alt-Delete, or when more than two keys are depressed because of fast typing ("rolling over" more than two keys).
"Multi-key rollover" is considered essential for quality keyboards and for English touch typing. This is where the most common key sequences have been studied and keys in the same common sequence are placed in the electrical switch matrix such that three keys down cannot produce a fourth "phantom" key by shorting out the matrix. The simplest way to accomplish this is to put all keys in the same common sequence on the same X or Y line of the switch matrix. As long as the electronics does not see more than two keys on different X and Y lines, which would create a phantom key, it will continue to process the next key depressed. This typically produces 4- to 5-key rollover for the most common key sequences.
Key blocking and ghosting 
Modern keyboards detect ghosting and instead of registering a fourth key will ignore the third key, which is known as blocking. Which keys jam when pressed together differs between brands and models of keyboards.
Most music keyboards and some high-end computer keyboards use an isolation diode with each keyswitch in their keyboard matrix. Such keyboards can correctly read any combination (chording) of keys pressed in any order and released in any order -- they are immune to both key ghosting and key blocking.
Key blocking is most often noticed when using a keyboard to play action-oriented computer games. The original Star Control game included a utility to test for key blocking and help the player determine the best key mapping for his/her keyboard, since during gameplay it was common for each of the two players to be pressing three or four keys at the same time.
Most modern action-oriented computer games and console emulators use the control, alt, and shift keys by default, since they are always designed to be pressed in combination with other keys, and thus usually circumvent key blocking.
See also 
- "Braille2000: Keyboard Requirements". Retrieved 2006-07-19.
- "Looking for a new keyboard?". 2004-10-26. Retrieved 2006-07-19.
- "TechEncyclopedia: n-key rollover". Retrieved 2011-12-03. mirror: "TechEncyclopedia: n-key rollover". Retrieved 2007-09-27.
- Dribin, Dave. "Keyboard Matrix Help, section 8: "Getting Rid Of Ghosting and Masking"". Retrieved 2008-10-11.