IBM PC keyboard

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Memorial to a keyboard in Yekaterinburg, Russia

The IBM PC or processing. In combination with Control, it produces a different keycode, for Break. Ctrl-Break traditionally stopped programs in DOS. Ctrl-Break is also used to halt execution of the debugger in some programming environments such as Microsoft Visual Studio. In combination with the Windows key, it brings up the System Properties window in Microsoft Windows environments.

Alt shifts the letters and numbers into the range above hex 0x80 where the international characters and special characters exist in the PC's standard character set. Alt plus a number typed on the numeric pad produces special characters, see Windows Alt keycodes.
AltGr works like the Ctrl+Alt key combination, often used to print special characters like the backslash on non-english keyboards. (On the original IBM AT Enhanced keyboard the right Alt key has green letters).
Fn may be present on compact keyboards such as those built into laptop computers. When depressed in combination with other keys, it either enables the user to access key functions that do not have dedicated keys on the compact keyboard (such as the numeric block), or it controls hardware functions such as switching between the built-in screen and an external display, changing screen brightness, or changing speaker volume. These alternate meanings are usually indicated with text or symbols of a different color printed on the key, with the 'Fn' key text having that same color.

[edit] Connectors

There are three types of connector used to connect a PC keyboard to the main system unit. All three are mechanically different from each other, but the first two are electrically identical (except for XT keyboards, which used a connector mechanically identical to the later AT connector, but not electrically compatible with it). The three connector types are listed below in chronological order: