Keyboard ghosting and rollover explained
Keys that stop registering when you hold several at once are usually a design limit, not a fault. Here is how to tell which one you have hit.
If keys stop registering when you hold several at once, but every key works perfectly on its own, nothing is broken. You have hit a design limit, and which limit it is determines whether anything can be done about it.
The two different limits
Rollover is how many keys your keyboard can report simultaneously. Press more than that and the extra ones are simply not sent. Nothing wrong happens, keys just stop appearing.
Ghosting is worse and rarer: a key you did not press registers, or a key you did press blocks another. This happens because of how the keyboard matrix is wired.
Keys are not connected individually. They sit at intersections of row and column traces, which is what keeps the wiring manageable. When three keys in a rectangle on that grid are held down, the controller cannot tell whether the fourth corner is also pressed, because the electrical path looks identical either way. Some keyboards report the phantom key anyway. Better ones detect the ambiguity and block it instead, which is called jamming and looks like rollover.
The fix in hardware is a diode on every key, so current only flows one way and the ambiguity disappears. That is what “n-key rollover” means physically, and it is why it cannot be added in software.
Finding your limit
Hold down as many keys as you can and watch the simultaneous count. It will stop rising at some point, and that number is your rollover.
| Peak count | What you have |
|---|---|
| 2 to 3 | A basic membrane keyboard, common on budget desktops |
| 6 | The standard USB HID limit, and a very common ceiling |
| 6 plus modifiers | Six-key rollover, adequate for most gaming |
| 10 or more | Effectively full n-key rollover |
Which specific combinations fail depends on the matrix wiring rather than on the keys themselves, which is why one set of three keys works and another set of three does not on the same board. That is normal and not a sign of a fault.
When it actually matters
For typing, rollover is close to irrelevant. Even fast touch typing rarely holds more than two or three keys simultaneously, because the previous key is released as the next is pressed.
For gaming it matters more. A worst case in a typical action game is three movement keys plus shift plus control, which is five. Six-key rollover covers almost everything. Rhythm games and flight simulators are where full n-key rollover earns its place.
The combination most people notice first is holding two movement directions plus jump or crouch, which is exactly the rectangle case that trips cheap matrices.
What this does not explain
If a key fails when pressed alone, that is a dead key and a different problem entirely. Rollover limits only ever affect simultaneous presses.
If a key produces the wrong character rather than nothing, that is a keyboard layout mismatch in your operating system, not a hardware limit.
And if keys stop registering only after the keyboard has been in use for a while, look at heat, a failing cable or a USB power setting rather than the matrix, since rollover limits do not change with time.
Common questions
Is ghosting the same as a broken key?
No, and the difference is easy to check. A broken key never registers on its own. A ghosting or rollover problem only appears when several keys are held together, and every key works perfectly when pressed alone.
What rollover do I actually need?
For typing, almost none: normal typing rarely holds more than two or three keys at once. For gaming, six is usually enough, since a typical worst case is three movement keys plus two modifiers. Full n-key rollover matters most for rhythm games and for people who use many-key shortcuts.
Why is my expensive keyboard limited over Bluetooth?
Some wireless implementations limit how many simultaneous keypresses they report to save bandwidth and power. If a keyboard shows full rollover over USB and less over Bluetooth, that is the wireless protocol rather than the keyboard matrix.
Can ghosting be fixed?
Not on a keyboard that has it, because it is a property of how the matrix is wired. The fix is a keyboard with diodes on each key, which is what n-key rollover means in hardware terms. No driver or setting adds it.
Does USB limit rollover to six keys?
The basic USB HID keyboard protocol reports six keys plus modifiers, which is where the common six-key figure comes from. Keyboards with full n-key rollover work around it by presenting themselves differently, so the limit is not absolute.