Key Ghosting, Blocking and Rollover (NKRO) Explained

Your keyboard may work perfectly one key at a time and still miss presses when several keys are held together. Here is why that happens, what the terms on the box mean, and how to test your own keyboard.

Guide · About 7 minutes to read

How a keyboard knows which key you pressed

Giving every key its own wire to the controller would need over a hundred connections. Instead, most keyboards arrange their switches in a grid called a matrix: each key sits where one row wire crosses one column wire. The controller scans the grid many times a second, powering one row at a time and checking which columns respond. A press closes the connection at that row and column, so the controller knows exactly which key it was.

This works perfectly for one key, and usually for two. The trouble starts when three or more keys are held in certain patterns.

Ghosting: keys you never pressed

Imagine holding three keys that sit at three corners of a rectangle in the matrix. With those three connections closed, current can flow along a path that makes the fourth corner look pressed too. The controller sees a key you never touched. That phantom press is called ghosting.

Blocking (or jamming): keys that get ignored

Ghost presses are worse than missing ones, so many keyboards are designed to detect the ambiguous pattern and ignore the extra key instead. That is blocking, sometimes called jamming: you press a key while holding others, and nothing happens. Boxes labelled "anti-ghosting" usually mean the keyboard blocks rather than ghosts, often only for a set of common gaming keys.

Rollover: how many keys at once

Key rollover is the number of keys that can be held at the same time and still all be reported correctly.

• 2KRO: any two keys always work together. Common on inexpensive membrane keyboards.
• 6KRO: up to six regular keys plus the modifiers (Shift, Ctrl, Alt, Windows/Command). This limit comes from the standard USB "boot" keyboard report, which has room for six key codes. It is enough for almost all typing and most games.
• NKRO (n-key rollover): every key can be held at once and all are reported. Keyboards that offer it usually have a diode on every switch, which stops current flowing backwards through the matrix and eliminates ghosting, and they use a different USB report that isn't limited to six keys.

Does rollover matter for you?

For everyday typing, rarely. Even fast typists seldom hold more than a few keys at once, because each key is released almost as soon as the next is pressed. Rollover matters more in games, where you might hold a direction, sprint, crouch and an ability key together, and in music or stenography software that relies on chords of keys.

How to test rollover in your browser

BoardCheck flashes a key every time the browser receives a press for it, so a press that the keyboard blocks simply never shows up:

1. Press and hold one key.
2. Without releasing it, press and hold a second, then a third, and so on.
3. Watch the board as you add each key. Every new key should flash as you press it.
4. If a new key doesn't flash, you have hit a blocking or rollover limit. Count how many keys you were already holding at that point.
5. If a key flashes that you are not touching, that is ghosting.

Combinations worth trying:

• W + A + Shift + Space (moving, sprinting and jumping in many games)
• Q + W + E + R together
• A S D F J K L ;, all eight home-row keys
• Keys in the same column, like 1 Q A Z, which share matrix wires on many boards

A few caveats

• Your operating system or browser may react to some combinations, for example Ctrl + W closes a browser tab. Avoid system shortcuts while testing.
• Bluetooth connections sometimes limit rollover even when the same keyboard does NKRO over a cable. Test the connection you actually use.
• Physical comfort matters too. If you can't hold ten keys at once, that's your hands, not your keyboard.

Summary

• Ghosting: an extra key appears that you didn't press.
• Blocking: a key you did press is ignored while others are held.
• 6KRO: six keys plus modifiers, plenty for most people.
• NKRO: every key, usually thanks to a diode per switch.

For a broader check that covers single keys, modifiers, chatter and the numpad, follow our complete keyboard testing checklist.