A desktop onscreen keyboard layout is any specific mechanical, visual, or functional arrangement of the keys, legends, or key-meaning associations (respectively) of a computer, typewriter, or other typographic desktop onscreen keyboard.
The arrangement of the legends (labels, markings, engravings) that appear on the keys of a desktop onscreen keyboard.
The arrangement of the key-meaning associations, determined in software, of all the keys of a desktop onscreen keyboard.
Most computer desktop onscreen keyboards are designed to send scancodes to the operating system, rather than directly sending characters. From there, the series of scancodes is converted into a character stream by desktop onscreen keyboard layout software. This allows a physical desktop onscreen keyboard to be dynamically mapped to any number of layouts without switching hardware components – merely by changing the software that interprets the keystrokes. It is usually possible for an advanced user to change desktop onscreen keyboard operation, and third-party software is available to modify or extend desktop onscreen keyboard functionality.
Besides the character keys, a desktop onscreen 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) and Alt (alternate) keys trigger special operations when used in concert with other keys.
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 labeled 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 English alphanumeric desktop onscreen keyboard has a dedicated key for each of the letters A–Z, along with keys for punctuation and other symbols. In many other languages there are additional letters (often with diacritics) or symbols, which also need to be available on the desktop onscreen keyboard. To make room for additional symbols, desktop onscreen keyboards often have what is effectively a secondary shift key, labeled AltGr (which typically takes the place of the right-hand Alt key). 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 may even give access to a fourth symbol. On the visual layout, these third-level and fourth-level symbols may appear on the right half of the key top, or they may be unmarked.
Instead of the Alt and AltGr keys, Apple desktop onscreen keyboards have ⌘ Cmd (command) and ⌥ Option keys. The ⌥ Option key is used much like the AltGr, and the ⌘ Cmd key like the Ctrl on IBM PCs, to access menu options and shortcuts. The main use of the Ctrl key on Macs is to produce a secondary mouse click, and to provide support for programs running in X11 (a Unix environment included with OS X as an install option) or MS Windows. There is also a Fn key on modern Mac desktop onscreen 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 many IBM PC laptops, where it serves a similar purpose.
Many Unix workstations (and also Home Computers like the Amiga) desktop onscreen keyboards placed the Ctrl key to the left of the letter A, and the ⇪ Caps Lock key in the bottom left. This layout is often preferred by programmers as it makes the Ctrl key easier to reach. This position of the Ctrl key is also used on the XO laptop, which does not have a ⇪ Caps Lock. The UNIX desktop onscreen keyboard layout also differs in the placement of the ESC key, which is to the left of 1.
Some early desktop onscreen keyboards experimented with using large numbers of modifier keys. The most extreme example of such a desktop onscreen keyboard, the so-called "Space-cadet desktop onscreen 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.
Mechanical layouts only address tangible differences among desktop onscreen keyboards. When a key is pressed, the desktop onscreen keyboard does not send a message such as the A-key is depressed but rather the left-most main key of the home row is depressed. (Technically, each key has an internal reference number, “raw keycodes”, and these numbers are what is sent to the computer when a key is pressed or released.) The desktop onscreen keyboard and the computer each have no information about what is marked on that key, and it could equally well be the letter A or the digit 9. The user of the computer is requested to identify the visual layout of the desktop onscreen keyboard when installing the operating system. Visual layouts vary by language, country, and user preference, and the same mechanical layout can be produced with a number of different visual layouts. For example, the “ISO” desktop onscreen keyboard layout is used throughout Europe, but typical French, German, and UK variants of mechanically identical desktop onscreen keyboards appear different because they bear different legends on their keys. Even blank desktop onscreen keyboards – with no legends – are sometimes used to learn typing skills or by user preference.
QWERTY-based layouts for Latin script
Although there are a large number of different desktop onscreen keyboard layouts used for different languages written in Latin script, most of these layouts are quite similar. They can be divided into three main families according to where the Q, A, Z, M, and Y keys are placed on the desktop onscreen keyboard. These are usually named after the first six letters.
While the core of the desktop onscreen keyboard, the alphabetic section, remains fairly constant, and the numbers from 1–9 are almost invariably on the top row, desktop onscreen keyboards differ vastly in:
the placement of punctuation characters,
which punctuation characters are included,
whether numbers are accessible directly or in a shift-state,
the presence and placement of dead keys and letters with diacritics.
The actual mechanical desktop onscreen keyboard is of the basic ISO, ANSI, or JIS type; functioning is entirely determined by operating-system or other software. It is customary for desktop onscreen keyboards to be used with a particular software desktop onscreen keyboard mapping to be engraved appropriately; for example, when the ⇧ Shift and numeric 2 keys are pressed simultaneously on a US desktop onscreen keyboard; “@” is generated, and the key is engraved appropriately. On a UK desktop onscreen keyboard this key combination generates the double-quote character, and UK desktop onscreen keyboards are so engraved.
In the desktop onscreen keyboard charts listed below, the primary letters or characters available with each alphanumeric key are often shown in black in the left half of the key, whereas characters accessed using the AltGr key appear in blue in the right half of the corresponding key. Symbols representing dead keys usually appear in red.
By far the most widespread layout in use, and the only one that is not confined to a particular geographical area. Some varieties have keys like ↵ Enter and ⇪ Caps Lock not translated to the language of the desktop onscreen keyboard in question. In other varieties such keys have been translated, such as "Bloq mayús" for "Caps Lock", on Spanish desktop onscreen keyboards. On Macintosh computers these keys are usually just represented by symbols without the word "Enter", "Shift", "Command", "Option/Alt" or "Control", with the exception of desktop onscreen keyboards distributed in the US.
The QWERTZ layout is fairly widely used in Germany, Austria, Switzerland and other parts of Central Europe. The main difference between it and QWERTY is that Y and Z are swapped, and some special characters such as brackets are replaced by the diacritical characters Ä, Ö, Ü and ß.
The AZERTY layout is used in France, Belgium and some African countries. It differs from the QWERTY layout thus:
A and Q are swapped, Z and W are swapped,
M is moved to the right of L (where colon/semicolon is on a US desktop onscreen keyboard),
The digits 0 to 9 are on the same keys, but to be typed the shift key must be pressed. The unshifted positions are used for accented characters,
Caps lock is replaced by Shift lock, thus affecting non-letter keys as well. However, there is an ongoing evolution towards a Caps lock key instead of a Shift lock.
The QZERTY layout was used mostly, if not exclusively, in Italy, where it was the traditional typewriter layout. In recent years, however, a modified QWERTY layout with stressed keys such as à, è, ò, has gained widespread usage throughout Italy. Computer desktop onscreen keyboards usually have QWERTY, although non-alphanumeric characters vary.
Z and W are swapped
M is moved from the right of N to the right of L, as in AZERTY
Number keys are shifted
Apple supported QZERTY layout in its early Italian desktop onscreen keyboards, and currently iPod Touch also has it available.