On the PC platform, the right Alt key has the print "Alt Gr" (Alt Graph) in European layouts and in "US International" layout, but "Alt" in US-English layout. It is the same key.
The key is interpreted differently in the host computer's operating system depending on which layout you set there in its settings. For German, for instance, it is "Alt Gr" and is used as a second-level shift key to get graphical characters. If you have your computer set for US-English, then it will work the same as the left Alt.
This stupidity is something that we got with the IBM PC platform, because the Alt key had been used as a command-key in MS-DOS programs for the English-speaking market.
On other platforms (such as Apple Mac, Amiga, Atari ST, RiscOS etc.), there was no need to split it up into two because the Alt (or "Option") key always produced graphical characters.