It's the right-alt key. If you're using a layout that defines AltGr, the key will just work like that on a US keyboard. Even with just the US keyboard layout, the right-Alt key can still be used with numbers in Windows to type any unicode character.
Which the parent was pointing out is nonstandard. There is no alt-gr key on a standard US keyboard. You can certainly remap alt-right to alt-gr but that is nonstandard and doing so does not mean that the key exists on the physical keyboard.
For example my caps lock key is mapped to ctrl-left. That is nonstandard.
* Pick any European keymap under Ubuntu, Windows, or Mac with your US keyboard * Hold right-alt and press a key
You'll get something like ń or ą.
Bonus: Under Ubuntu, you might also have other keys remapped to let you type things like x²≠¼.
Bonus hack: Add a Greek keymap, and you can even get to things like Δx = π·y with an alt-space in between.