Mode keys change the actions of other keys (or other input devices). There are two kinds of mode keys: toggle keys and modifier keys.
A toggle key turns a particular mode on or off each time it is pressed. For example, pressing the CAPS LOCK key toggles uppercase alphabetic keys; pressing the NUM LOCK key toggles between numeric and directional input using the keypad keys.
Like toggle keys, modifier keys change the actions of normal input. Unlike toggle keys, however, modifier keys establish modes that remain in effect only while the modifier key is held down. Modifier keys include the SHIFT, CTRL, and ALT keys. Such a "spring-loaded" mode is often preferable to a "locked" mode because it requires the user to continuously activate it, making it a conscious choice and allowing the user to easily cancel the mode by releasing the key.
Because it can be difficult for a user to remember multiple modifier assignments, avoid using multiple modifier keys as the primary means of access to basic operations. In some contexts, such as environments that are specific to pen input, the keyboard may not be available. Therefore, use modifier-based actions only for quick access to operations that are supported adequately elsewhere in the interface.