Keyboard Ghosting, Rollover and NKRO Explained

What keyboard ghosting is, how the switch matrix causes it, what 2KRO, 6KRO and NKRO mean, why USB vs PS/2 is outdated, and how to test your own keyboard.

If you have ever held two keys and found a third refused to register, or seen a game character do something you did not press, you have run into the limits of how keyboards read their switches. This guide explains the matrix behind it, what the rollover numbers on a keyboard's box actually promise, and how to test your own keyboard in the browser.

Run the rollover test

Why keyboards use a matrix

A full-size keyboard has around 104 switches. Giving each switch its own wire to the controller would need 104 inputs, so keyboards wire the switches into a grid instead: typically 8 rows and around 18 columns. The controller drives one row at a time and reads all the columns; a pressed switch connects its row to its column, so the controller sees which switches are down by scanning rows in sequence a few hundred to a few thousand times per second. About 26 pins cover more than 100 keys.

The saving comes with a catch. When several switches are pressed, current has more than one path through the grid. Press three keys that form three corners of a rectangle in the matrix (the same two rows and the same two columns) and current can flow from the driven row, through the first switch, along the column, through the second switch onto the other row, and through the third switch back onto a column the controller is reading. The fourth corner of the rectangle looks pressed even though nobody touched it. That phantom keypress is a ghost.

Blocking, ghosting, and diodes

Cheap keyboards handle this in firmware: when the controller detects a combination that could produce a ghost, it simply refuses to report the third key. That is called blocking or jamming, and it is why a laptop might type A and D together but drop W when you add it. The key you pressed does nothing, which is safer than a ghost but still a failure in a game.

The proper fix is a diode in series with every switch. A diode lets current flow in one direction only, so the sneak path back through the second switch is cut off and the controller can read any combination of switches without ambiguity. Almost every mechanical keyboard has per-key diodes; almost no membrane or laptop keyboard does, because it adds 100 components to a product built to a price. Manufacturers who advertise "anti-ghosting" on a membrane board usually mean they have added diodes to the WASD block and a few common gaming keys only.

2KRO, 6KRO, and NKRO

Rollover is the number of keys a keyboard can register at once and report correctly. The number after the K is that count.

• 2KRO guarantees any two keys. Three may or may not work depending on where they fall in the matrix. Typical of basic membrane and laptop keyboards.
• 6KRO guarantees six non-modifier keys plus Shift, Ctrl, Alt, and the OS key, which are reported separately. This is the limit of the USB "boot protocol" report format, which was designed so a keyboard would work with a BIOS before any driver loaded. Many mechanical keyboards ship in this mode.
• NKRO means every key can be down at once and each one is reported. It needs per-key diodes on the hardware side and a different USB report format on the firmware side.

Anti-ghosting and rollover are related but not the same. A keyboard can have full diodes (no ghosts possible) but still be limited to reporting six keys by its USB mode. Conversely, a membrane board might promise "26-key anti-ghosting" for the gaming cluster and only 2KRO everywhere else.

Why WASD, Shift, and Space matter

The combinations that break in games are predictable. Sprinting diagonally while jumping means holding W, A, Shift, and Space at the same time, and then perhaps tapping R to reload or E to interact. On a 2KRO membrane board, W and A frequently sit in a matrix rectangle with Space or with a nearby letter, so one of them is blocked and the character stops moving or fails to jump. Shift is a modifier and does not count against rollover on most boards, but on some very cheap keyboards Shift is in the matrix like any other key and adds to the problem.

Fighting games with multi-button inputs, rhythm games where you hold chords, and emulators with two players on one keyboard push past six keys quickly, which is where 6KRO stops being enough and NKRO is worth having. For typing, spreadsheets, and shortcuts, rollover never becomes a problem, because a fast typist holds at most three keys for a few milliseconds.

The USB vs PS/2 myth

You will still read that "USB is limited to six keys, so real NKRO needs PS/2". That was true of early USB keyboards and is now wrong. PS/2 sends each keypress as its own interrupt to the computer, so it has no inherent limit. USB's boot protocol report has six slots, but a keyboard does not have to use the boot protocol once the operating system has loaded. NKRO USB keyboards send a bitmap report with one bit per key (or several boot-protocol reports on multiple endpoints), and Windows, macOS, and Linux have all understood this for many years. If your USB keyboard tops out at six, look for a mode switch; the interface is not the cause. PS/2 ports are now rare enough that the question is mostly academic.

How the tester measures simultaneous keys

The keyboard tester listens for keydown and keyup events from the browser and keeps a set of keys currently held. The rollover section reports the largest number of keys it ever saw down at the same moment. To test, place several fingers on the keyboard and press keys together, then add keys one at a time without releasing any. Watch the held-key count climb:

1. Press W, A, and Space together. If all three light up, the gaming cluster is at least 3KRO, which covers most games.
2. Add Shift, Ctrl, E, and R. Modifiers usually appear regardless; watch whether the letter keys stop registering.
3. Roll all ten fingers onto the home row and the row above. A count that stops at exactly 6 (plus modifiers) means 6KRO or boot-protocol mode. A count that keeps rising past 10 means NKRO is working.
4. Try three keys that form a rectangle in a typical matrix, such as Q, W, and A, then Z. On a keyboard without diodes the fourth key may light up on its own, which is a genuine ghost.

A few caveats: the operating system swallows some combinations (Alt+Tab, the Windows key with certain letters, Ctrl+Alt+Del) before the browser sees them, so a missing key in those cases is not a keyboard fault. Some laptops and Apple keyboards limit certain combinations at the firmware level. And if you cannot reach six keys at all, check the tester's timing panel and the key not working guide; a dead switch and rollover blocking look identical from the outside.

What to buy if it matters

For general use, any 6KRO keyboard with diodes is enough. For gaming, look for "NKRO" or "full N-key rollover" rather than "anti-ghosting", since the latter usually covers only a subset of keys. Be aware that some NKRO boards need a toggle to leave boot mode, and that NKRO over Bluetooth is often limited to 6KRO by the wireless firmware even when the same keyboard does NKRO over the cable. If you already own the keyboard, the rollover test above tells you what you actually have in under a minute.

Last updated 2026-09-16. Everything on this page runs in your browser. Nothing you type or upload leaves your device.