AltGr is used similarly to the Shift key: it is held down when another key is struck in order to obtain a character other than the one that the latter normally produces. AltGr and Shift can also sometimes be combined to obtain yet another character. For example, on the US-International keyboard layout, the C key can be used to insert four different characters:

C → c (lower case — first level)
⇧ Shift+C → C (upper case — second level)
AltGr+C → © (copyright sign — third level)
AltGr+⇧ Shift+C → ¢ (cent sign — fourth level)

The Alt Gr key is something that was used in the early days of computing but is no longer of much use, but it’s still there because nobody has thought about removing it.

Our standard computer keyboards are based on the design of computer keyboards from the 1960s and 1970s when, rather than being used with desktop and laptop PCs, they were used to control giant mainframe computers that were the size of rooms and had thousands of functions.

There is no mouse control on a mainframe so if you wanted to get anything done you would have to do it using the keyboard, and so to enable people to work quickly keyboards were covered with lots of symbols.

Sometimes two or three functions were accessed using a single key and to access them, the user would have to press another key such as Shift, Ctrl, Alt or Alt Gr. The Alt Gr originally stood for ‘alternate graphic’ and if you pressed it the function of the key would change to what was shown in the alternative graphic on the top of the key.

Today, with Windows computers, most of us use a mouse to access what we need from the computer so there is no longer any need for a keyboard to be covered with numerous arcane symbols.

The Shift, Ctrl and Alt keys are still there and are still used by lots of programs. The Alt Gr key is almost never used now, but remains on modern keyboards because it is still used by some older applications.

Also, having a single keyboard design allows keyboards to be used on all sorts of computers, some of which (older ones) still use the Alt Gr key.

That said, the Alt Gr key does have a use for modern Windows users, because it can be used to type some special characters such as those from other languages.