N-key rollover and ghosting
Ghosting is one of the most misused terms in keyboard marketing. It describes a specific electrical failure that essentially does not occur on any mechanical keyboard made in the last fifteen years, and the thing people usually mean instead is a different, milder problem.
Real ghosting, and why it is extinct
Keyboards are wired as a matrix of rows and columns to avoid needing a wire per key. Pressing a key connects one row to one column, and the controller works out which key that was.
Without isolation, three simultaneous presses in a rectangle can create a path through the matrix that looks identical to a fourth key being pressed. The controller has no way to distinguish it, so it reports a key you never touched. That is ghosting.
The fix is a diode on every switch, forcing current in one direction and eliminating the phantom path. Every mechanical keyboard has these. Ghosting was a real problem on cheap membrane keyboards, it is why the word entered gaming vocabulary, and it is not something you need to consider when buying a mechanical board.
Blocking, which is what people actually encounter
USB's basic keyboard protocol reports up to six non-modifier keys at once, plus the eight modifiers. If you press a seventh, something has to give: the keyboard either drops the new key or drops an old one.
This is a reporting limit, not an electrical fault, and it is much less dramatic than ghosting: you press a key and nothing happens, rather than a key you did not press firing.
| Scenario | Keys held | Problem? |
|---|---|---|
| Strafing, crouching, shooting, pinging | 4 | No |
| Diagonal movement, crouch, walk, ability, ability | 6 | At the limit |
| Rhythm games | Often 7 or more | Yes |
| Held modifiers plus a chord | Modifiers do not count towards the six | No |
NKRO over USB
N-key rollover on a USB keyboard is achieved by presenting a different report format that encodes a bitmap of every key rather than a list of six. This works fine in any modern operating system.
The catch is boot environments. Some BIOS and UEFI implementations only understand the basic six-key boot protocol, so a keyboard in NKRO mode may not work before the operating system loads. This is why many boards ship in 6KRO by default with a toggle, often a key combination, to switch.
Practical advice: enable NKRO, then test that you can enter your BIOS. If you can, leave it on. If not, you have learned something about your motherboard and can switch back with essentially no loss.
The marketing to ignore
"Anti-ghosting" on a mechanical keyboard is not a feature, it is a description of having diodes, which every mechanical keyboard has. When a specification sheet advertises "anti-ghosting on 26 keys", it is describing a partial rollover implementation using marketing language borrowed from a different problem. Look for the phrase "N-key rollover" or "NKRO" instead.
Related reading
- Keyboard latency, including matrix scan rate.
- Switch types and the database.
- Glossary for the rest of the vocabulary.
Frequently asked questions
What is keyboard ghosting?
Genuine ghosting is when pressing two keys causes the keyboard to report a third key that you did not press. It happens on matrix keyboards without isolation diodes, because current can flow backwards through the matrix and complete an unintended circuit. Every mechanical keyboard has a diode on every switch, so real ghosting does not occur on them.
What is key blocking?
The thing people usually mean by ghosting: pressing a certain combination of keys causes one of them to be ignored. It happens on keyboards limited to reporting a fixed number of simultaneous keys, and it is a reporting limit rather than an electrical fault.
What does NKRO mean?
N-key rollover: the keyboard can report any number of simultaneously pressed keys correctly. Over USB it requires the keyboard to present itself in a mode beyond the basic boot protocol, which is why some boards have an NKRO toggle and default to 6-key mode for BIOS compatibility.
Is 6-key rollover enough for gaming?
Usually. Six simultaneous keys plus all modifiers covers nearly every realistic in-game combination: two movement keys, crouch, a weapon, an ability and a ping. You reach the limit in rhythm games, when using many held modifiers, or with certain macro-heavy setups.
Do I need to enable NKRO?
If your keyboard offers it and works in your BIOS, yes, turn it on and forget about it. The default 6KRO exists for compatibility with systems that only understand the USB boot keyboard protocol. If your machine cannot see the keyboard at boot after enabling it, switch back.