It is easy to type accented characters (like ä, the letter “a” with two dots placed over it) using the virtual keyboard of your iPhone and iPad. Just tap and hold a key and it will show you a list of related characters that have accent marks.
The passcode of your iPhone /iPad is generally made up of digits but if you would like to use a more strong and un-guessable passcode, consider using one or more of these accented characters. For instance, a word like Äpplë is much more difficult to guess than just Apple.
If you do not see the option to use alphanumeric characters as the passcode for your iPhone or iPad, go to Settings -> General -> Passcode Lock and turn off Simple Passcode.
Also see: Keyboard Shortcuts for iPhone & iPad
This is an iOS specific feature. Android devices support alphanumeric characters for screen passwords but accented characters aren’t allowed. And in the case of Windows Phone 8, you can only use digits to lock /unlock the screen.
The tip was originally shared by @ade494 on Reddit.