page_title: Keyboard layout
section_title: Modifier keys
hierarchical_section_title: Keyboard layout / Key types / Modifier keys
caption_attribution_description: Symbolics LM-2 Lisp machine space-cadet keyboard
context_page_description: A keyboard layout is any specific mechanical, visual, or functional arrangement of the keys, legends, or key-meaning associations of a computer, typewriter, or other typographic keyboard.
  Mechanical layout is the placements and keys of a keyboard. Visual layout is the arrangement of the legends that appear on the keys of a keyboard. Functional layout is the arrangement of the key-meaning association or keyboard mapping, determined in software, of all the keys of a keyboard.
  Modern computer keyboards are designed to send a scancode to the operating system when a key is pressed, which is not necessarily the specific character engraved on that key. The OS converts the scancode into a specific binary character code using a "scancode to character" conversion table, called the keyboard mapping table. This means that a physical keyboard may be dynamically mapped to any layout without switching hardware components – merely by changing the software that interprets the keystrokes. Often, a user can change keyboard mapping in system settings. In addition, software may be available to modify or extend keyboard functionality.
context_section_description: Besides the character keys, a keyboard incorporates special keys that do nothing by themselves but modify the functions of other keys. For example, the ⇧ Shift key can be used to alter the output of character keys, whereas the Ctrl (control), Alt (alternate) and AltGr (alternative graphic) keys trigger special operations when used in concert with other keys. (Apple keyboards have differently labelled but equivalent keys, see below).
  Typically, a modifier key is held down while another key is struck. To facilitate this, modifier keys usually come in pairs, one functionally identical key for each hand, so holding a modifier key with one hand leaves the other hand free to strike another key.
  An alphanumeric key labelled with only a single letter (usually the capital form) can generally be struck to type either a lower case or capital letter, the latter requiring the simultaneous holding of the ⇧ Shift key. The ⇧ Shift key is also used to type the upper of two symbols engraved on a given key, the lower being typed without using the modifier key.
  The Latin alphabet keyboard has a dedicated key for each of the letters A–Z, keys for punctuation and other symbols, usually a row of function keys, often a numeric keypad and some system control keyns. In most languages except English, additional letters (some with diacritics) are required and some are present as standard on each national keyboard, as appropriate for its national language. These keyboards have another modified key, labelled AltGr (alternative graphic), to the right of the space-bar. (US keyboards just have a second Alt key in this position). It can be used to type an extra symbol in addition to the two otherwise available with an alphanumeric key, and using it simultaneously with the ⇧ Shift key usually gives access to a fourth symbol. These third-level and fourth-level symbols may be engraved on the right half of the key top, or they may be unmarked.  Cyrillic alphabet and Greek alphabet keyboards have similar arrangements.
  Instead of the Ctrl, Alt and AltGr keys seen on commodity keyboards, Apple Keyboards have ⌘ Cmd (command) and ⌥ Option keys. The ⌥ Option key is used much like the AltGr, and the ⌘ Cmd key like the Ctrl and Alt, to access menu options and shortcuts. Macs have a Ctrl key for compatibility with programs that expect a more traditional keyboard layout. It is especially useful when using a terminal, X11 (a Unix environment included with OS X as an install option) or MS Windows. The key can generally be used to produce a secondary mouse click as well. There is also a Fn key on modern Mac keyboards, which is used for switching between use of the F1, F2, etc. keys either as function keys or for other functions like media control, accessing dashboard widgets, controlling the volume, or handling exposé. Fn key can be also found on smaller Windows and Linux laptops and tablets, where it serves a similar purpose.
  Many Unix workstations (and also home computers like the Amiga) keyboards placed the Ctrl key to the left of the letter A, and the ⇪ Caps Lock key in the bottom left. This position of the Ctrl key is also used on the XO laptop, which does not have a ⇪ Caps Lock. The UNIX keyboard layout also differs in the placement of the ESC key, which is to the left of 1.
  Some early keyboards experimented with using large numbers of modifier keys. The most extreme example of such a keyboard, the so-called "Space-cadet keyboard" found on MIT LISP machines, had no fewer than seven modifier keys: four control keys, Ctrl, Meta, Hyper, and Super, along with three shift keys, ⇧ Shift, Top, and Front. This allowed the user to type over 8000 possible characters by playing suitable "chords" with many modifier keys pressed simultaneously.
caption: MIT "space-cadet keyboard", an early keyboard with a large number of modifier keys. It was equipped with four keys for bucky bits (Control, Meta, Hyper, and Super); and three shift keys, called "shift", "top", and "front".
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