Press any key — it lights up below with its exact code, and the counter shows how many keys your board reports at once (rollover). The full tester draws your whole layout and tracks every key you've hit.
Dead keys: press every key once — anything that doesn't register is a hardware or connector fault. Rollover (NKRO): hold several keys at once; gaming boards report 6 to unlimited simultaneous keys, cheap membrane boards often collapse at 3–4, which in games means dropped inputs while strafe-jumping. Ghosting: on low-end matrices, pressing three keys can register a phantom fourth. Chatter: a worn switch fires twice per press — if a key double-types, that switch (or its debounce) is failing.
Browser caveat worth knowing: the OS eats some combinations before any web page sees them (Alt+Tab, Ctrl+W, media keys on some layouts). A key that "fails" only in combination with system shortcuts is usually the OS, not the board.
First rule out the OS: shortcuts like Ctrl+W or Alt+F4 are intercepted by the browser or system before the page sees them. If a key registers nowhere — including a text editor — it's the keyboard: switch, solder joint or cable.
Rollover is how many simultaneous presses the board reports. 6-key rollover covers nearly all gaming; N-key (unlimited) is nice for rhythm games and heavy modifier use. Membrane boards that drop to 3–4 will eat inputs mid-fight.
A single press registering twice, caused by a worn or dirty switch bouncing. On hot-swap boards, replace the switch; otherwise firmware debounce settings or contact cleaner can help.