There are many features that keyboard input can require. These include:
1. Single character keystrokes
Obviously the main stay of many of the keyboard layouts, a simple 1-1 mapping of keystrokes to characters is what the bulk of most keyboard layout will consist of. Some languages will use many other features as well, but all of them are likely to have at least a few of the single character keystrokes.
2. Ligatures
There are many times that a single keystroke needs to enter more than one character. In keyboard nomenclature, these 1:many mappings are called ligatures. Note that this definition of ligature is not identical to the one used in typography or in language orthographies; "ligature" here is used to identify multiple UTF-16 code points that are input by a single keystroke. This could be used in a number of ways: to represent a linguistic character consisting of multiple UTF-16 code points (such as Sri and Ksa seen on the Tamil keyboard); to represent multiple linguistic characters which often work together in the language; or to develop a keyboard layout to handle a language represented by supplementary characters (such as the Deseret keyboard)
3. Dead keys
The dead key mechanism is either very intuitive or incredibly confusing. The basic concept is that you type a character defined on the particular keyboard as a dead key, then type a specific second character known as a base character. Rather than displaying these two characters, a unique third character known as a combining character will be shown. The reason the first character is defined as a "dead" key is
4. Shift states
A keyboard layout typically has only 47 or 48 assigned physical keys on it; even the English alphabet would not fit, if you wanted both uppercase and lowercase A to Z (there wouldn’t even be room for punctuation characters). Therefore, keyboards usually contain another set of 47 or 48 keys that can be accessed by pressing Shift in tandem with a character (for examples, see Figure 12 and 13 for the Greek keyboard in both the unshifted and shifted states).
5. AltGr shift states
Some languages need more than 96 keys to input their language properly. Using just the shift state is not sufficient, so an additional shift state is added when Control+Alt is pressed. A shortcut to this key combination is to use the Right Alt key, also known as the AltGr key. This behavior is only expected for the keyboard layouts that define characters in the Control+Alt shift state. An example of this is the Polish keyboard layout (see Figures 14-16 for the unshifted, shifted, and Alt+Gr states of this keyboard).
6. Control shift states
While it is technically possible to use the Control (CTRL) key as a shift character as well, it is highly discouraged. The reason is that many programs use the CTRL key for various command functions (such as Ctrl+S to mean "Save...") and many times if keystrokes are assigned in the keyboard layout, those keystrokes will not work properly in programs that specifically handle them for other purposes.
7. Caps Lock key
The caps lock key is usually intended to be a version of the shift key that (a) only shifts characters that are cased versions of each other, and (b) stays shifted without having to hold down the key. On keyboard layouts for languages without a notion of case, the caps lock may do nothing, or it may be used for some other purpose entirely.
8. SGCap shift states
Some keyboards use the Caps Lock key as an access point for an entirely independent shift state for some of the keys. Originally named for its use in the "Swiss German" keyboard, the SGCaps shift state is also used in the Czech and Hebrew keyboards to allow this extra shift state. Like dead keys, they are either very intuitive if you are used to them and incredibly confusing if you aren’t familiar with them. The only real distinction of the SGCap shift states is that the Caps Lock key opens one to two entirely new shift states (an additional 96 characters, between the shifted and unshifted state). Using SGCap shift states in any other keyboards is discouraged unless you want a keyboard layout to have the same feel as one of the keyboards that uses the functionality.
9. Extended shift states
It is technically possible to add up to three additional keys as "Shift" keys. When combined with all possible combinations of the other shift keys this would allow a total of 55 other shift states. This feature is not used in any keyboards to its fullest extent; the Canadian Multilingual Standard keyboard layout is the only one that uses even a single extended shift state.