Keyboard ghosting and rollover, explained

Ghosting is when a keyboard reports a key you never pressed, because of the keys you are already holding. Most keyboards prevent it by ignoring the new press instead, which is called blocking: you hold W, A and Space, press Shift, and nothing happens. Gamers call both ghosting, and both come from how the keys are wired. Hold the keys you use together on the keyboard test to see which combinations your keyboard reports.

Keyboard test

Keys sit where row and column wires cross, and the keyboard finds a press by checking each crossing. Hold three keys on the corners of a rectangle and current can run through them to the fourth corner, so that key reads as pressed. A diode on each key lets current pass one way only, so every key reads correctly.

The terms in plain words

Ghosting: the keyboard reports a key nobody pressed.
Blocking (also called jamming): the keyboard ignores a key you pressed, to avoid a ghost. Most "ghosting" in games is really this.
Key rollover (KRO): how many keys you can hold at once with every one reported. 6KRO means any six; NKRO (n-key rollover) means every key at once.
Anti-ghosting: a maker's word, not a standard. It can mean the keyboard blocks ghosts, or that a group of keys, often W, A, S, D and the keys around them, works in any combination. It does not promise NKRO; the rollover figure in the specifications does.

Test your keyboard

Open the keyboard test. Each key you hold lights up, and Held now and Maximum together count them.
Press the keys you use together in games one at a time, without letting go: for example W, then A, then Shift, then Space.
If a key you add does not light up, the keyboard blocked that combination. If a key lights up that you never touched, that is a ghost.
Let go and try other groups: W, D and Space, the number keys with Shift, the arrow keys with Ctrl. Note which ones fail.

The test shows what reaches your browser. The system keeps some combinations for itself, such as Alt with Tab or anything with the Windows or Command key, and most laptops handle the Fn key inside the keyboard, so those keys tell you nothing about rollover.

Which combinations matter in games

Movement plus one or two actions: W or S with A or D, held with Shift to sprint or Ctrl to crouch, then Space to jump and Q, E, R or a number key. That can be five keys at once. A keyboard with 6KRO or better handles any of them; a basic office keyboard may block some, depending on how its wiring groups the keys. Rhythm games and two players sharing one keyboard need the most keys together.

Why six keys: the USB limit

The simplest USB keyboard report, the one a computer's startup screen understands, carries six ordinary keys plus the modifier keys (Ctrl, Shift, Alt and the Windows or Command key, left and right). A keyboard that only sends that report stops at six keys at once. NKRO keyboards send a larger report; many have a switch between NKRO and 6KRO because some startup screens and KVM switches only understand the small one.

What to do about it

Move the action. If one combination fails, bind that action to a key that works with the others; the test shows which.
Turn on NKRO. Many gaming keyboards have an NKRO mode, switched on in their app or with a key combination; the manual says how.
Plug it in directly. A KVM switch can limit a keyboard to the six-key report. Test it plugged straight into the computer.
Buy by the rollover figure. Look for NKRO, or the number of keys reported at once, rather than the word anti-ghosting.

Keys typing twice instead? Read why a keyboard types double letters. Not sure which keyboard shape you have? See ANSI vs ISO keyboards.