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.

> If you're using a layout that defines AltGr

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.

You're wrong:

* 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.

AFAIK neither Windows nor X11 have a concept of an AltGr key - it's just the right alt key so there is nothing to remap. The only difference between US and european layouts is its use as a modifier key.