Ghosting, N-Key Rollover, Chatter and Double Presses
The USB HID boot protocol carries 6 regular keys plus the modifier byte in a single report, which means a keyboard can be working exactly as designed and still refuse a seventh key. Four different problems get called ghosting in casual use, and they have four different causes, four different symptoms and four different answers.
Ghosting, in the original sense
A keyboard is a grid of rows and columns, and the controller works out which keys are down by scanning it. Without an isolating diode at each intersection, certain three key combinations complete a circuit through a fourth intersection that nobody pressed, and the controller reports a key that was never touched. That phantom character is what ghosting originally meant.
It is now rare, because the cure is cheap. Manufacturers who did not want to fit diodes chose the other option instead.
Blocking, which is what people actually see
The cheaper cure is to refuse ambiguous combinations rather than guess them. When the controller detects a pattern that could produce a phantom, it drops the third key instead of inventing a fourth. Nothing wrong appears on screen, and one of the keys you are holding simply does nothing.
In games this is the familiar complaint: hold two movement keys, add a third action, and the third one is ignored. In typing it almost never appears, because ordinary text rarely holds more than a letter and a modifier at the same time. That difference in exposure is why gamers discover the limit of a board and touch typists usually do not.
N-key rollover, and the six key limit
N-key rollover means every key has its own path to the controller, so any number of them can be held together and all of them register. It is a property of the matrix and its diodes.
Reporting is a separate question. The boot protocol allows six, so a keyboard using it tops out at six regular keys plus Shift, Ctrl, Alt and the operating system key, which continue to work because they travel in their own byte. Boards that advertise full rollover over USB use a different report format to get around that ceiling, and some offer a switch between the two for compatibility with firmware that only understands the simple one.
Six keys plus modifiers is more than typing ever needs. Anyone counting keys because a game feels unresponsive is looking at the right limit; anyone counting them because of a missed character in a document is looking at the wrong one.
The limit is a property of the connection rather than of keyboards in general. The older PS/2 interface sent each change of state as it happened instead of packing a fixed number of keys into a report, so full rollover came free, which is why adapters and legacy boards sometimes behave better in this one respect than modern hardware does.
Chatter and double presses
Chatter is a single press that registers more than once. Mechanical contacts bounce as they close, firmware waits a few milliseconds to ignore that bounce, and a switch whose contacts have worn or picked up contamination bounces for longer than the wait allows. The result is hhello from one deliberate press.
Auto repeat looks similar and is not a fault. Hold any key and the operating system repeats it on purpose, and a browser marks those events with the repeat flag set, which is exactly how a test tells the two apart. A genuine double from a single tap arrives without that flag.
Contamination is the common cause, so the first thing to try is not a replacement switch. Cleaning the board settles a fair number of chattering keys, and it costs nothing.
Checking each one in a browser
The keyboard test covers all four, and each needs a different action.
1. For blocking, hold three or four keys together and watch which ones light up. Repeat with different combinations, since the pattern depends on the matrix. 2. For the reporting ceiling, hold keys one at a time until a new one stops appearing, then count. 3. For chatter, tap a suspect key slowly twenty or thirty times and watch for a press you did not make. 4. For a dead intersection, press every key once and look for the one that never lights.
One caution about interpreting results. A key that produces nothing on its first press may be behaving correctly, because dead keys are designed to wait for a second press before printing anything. Faults live in the difference between what you pressed and what the board reported, not in what the screen shows.
None of this needs a stressful test to confirm. Once the board is behaving, Alfred Tennyson's The Brook, published in 1855, is a calm way to use it: 482 characters in which short verbs repeat in pairs, so a doubled letter or a missing one shows itself. William Blake's Laughing Song, from the Songs of Innocence he issued in 1789, repeats the same line pattern across 350 characters, which is the same check in fewer keystrokes. And Walt Whitman's There was a child went forth every day, printed in the first Leaves of Grass in 1855, is 225 characters, short enough to copy twice and notice whether every letter arrived exactly once. All three are waiting in the typing test, each with its author, year and source on its own page.