How to test for ghosting
Hold down three keys at once in the tester above, then a fourth, then a fifth, keeping all of them down. Watch the keys-at-once counter. On a keyboard with no limitation it keeps climbing. On a typical membrane keyboard it stops somewhere around two or three for keys in the same region of the board.
Try the combinations you actually use. For gaming that usually means W, A, Shift and Space held together, or the arrow keys with Control. For typing it rarely matters at all, which is why most people never discover their keyboard has a limit.
Keys that stop reporting while held are being blocked. Keys that appear without being pressed are true ghosting, which is far rarer on modern hardware.
What ghosting actually is
Keyboard switches are wired in a grid of rows and columns rather than each having its own wire to the controller, because a separate wire per key would need over a hundred connections. The controller scans the grid and works out which intersections are closed.
With three keys held down in a rectangle on that grid, current can travel an unintended path and make a fourth intersection look closed. The keyboard reports a key nobody pressed. That is ghosting in the original sense, and it is why the term exists.
Manufacturers fixed it cheaply by making the controller refuse ambiguous combinations instead of guessing. That is blocking, sometimes called jamming: rather than inventing a key, the keyboard drops one. Almost everything people call ghosting today is really blocking, and this test shows it as a key that goes quiet while you are still holding it.
Why some boards have no limit
Putting a diode behind every switch stops current taking the unintended path, which removes the ambiguity entirely and lets the controller read any number of simultaneous presses. Keyboards built this way are described as having N-key rollover.
Diodes cost money and board space, so they are the first thing removed from a budget membrane keyboard and standard on mechanical boards aimed at gaming. That is the real difference behind the marketing term anti-ghosting.
What you can do about it
- Remap the combination. If Shift, W and Space block each other, moving crouch to Control may sidestep the dead rectangle entirely.
- Check whether the board has a gaming or NKRO toggle in firmware or its own software. Some keyboards ship in a compatibility mode with a six-key limit and switch to full rollover on request.
- If the keyboard is USB, a six-key ceiling may be the USB report format rather than the hardware, which is covered on the rollover page.
- If none of that helps, the limit is in the matrix and no software will remove it.