I am working on a chat application and it is a web app. I tried mapping few keyboard shortcuts to Alt + modifier in Windows. To my knowledge, I didn't find any application using only Alt as a modifier. Can we use it? If not, why?
It really differs for Mac and Windows platforms.
On Windows you'd usually use Alt-combos with alphanumeric characters as "access keys"
Access keys have the following characteristics:
They use the Alt key plus an alphanumeric key.
They are primarily for accessibility.
They are assigned to all menus and most dialog box controls.
They aren't intended to be memorized, so they are documented directly in the UI by underlining the corresponding control label character.
They have effect only in the current window, and navigate to the corresponding menu item or control.
They aren't assigned consistently because they can't always be. However, access keys should be assigned consistently for commonly used commands, especially commit buttons.
They are localized.
Because access keys aren't intended to be memorized, they are assigned to a character that is early in the label to make them easy to find, even if there is a keyword that appears later in the label.
While Alt-combos with non-alphanumerics are for shortcuts:
By contrast, shortcut keys have the following characteristics:
- They primarily use Ctrl and Function key sequences (Windows system shortcut keys also use Alt+non-alphanumeric keys and the Windows logo key).
On Mac they call Alt key "an Option key" (it's functionally different, but Windows keyboards used on Mac will use Alt as "Option") and it's use is rather different - you should use it sparingly to give the user a special third option on shortcut (e.g., [Cmd]+[Q] is "Quit the app", [Shift]+[Command]+[Q] is "Log out the current user" and [Option]+[Shift]+[Command]+[Q] is "Log out the current user without confirmation.") and mostly for power user functionality:
Use the Option key as a modifier sparingly. If a third, less-common command is related to a pair of commands that use Command and Shift-Command, you can use Option-Command in the third command’s shortcut. Use this combinations very rarely. You can also use Option in a shortcut to a convenience or power-user feature. For example, Finder uses Option-Command-W for Close All Windows and Option-Command-M for Minimize All Windows.
As much as possible, avoid using the Control key as a modifier. The Control key is already used extensively throughout the system. To avoid possible conflicts, use it only when necessary.
Also, Alt (or Option) key is often used my different languages as a diacritic modifier, for example, is some Latvian keyboard layouts to get to "ā" you go [Alt]+[a], so it's something to consider when designing your shortcuts.
You Can! But if you want to know the real paradigm of the user, shortcuts with Alt is not very common and not easily predictable from the point of view of typical users. So, considering that, it might not be a good choice to add those shortcuts, rather choose something that can be intuitive for the user from the beginning.
If you can provide a nice guided tour to make the user being familiar with the keyboard shortcuts, you can make the user focus on your keyboard shortcuts. A good guided tour on the starting point can easily make a mental model of how to use the software without not guessing much.
Hope that helps :)