It's AltGr, which is on the right side. And that makes it quite awkward to use. I either break half my hand or need both hands. Or resort to Ctrl+Alt on the left side which isn't optimal either ...
They are? Every time I want to access a menu or a control in a dialog by keyboard I need Alt. Also in Far Alt+typing searches which I use frequently to get around the file system. Sure, one could get used to doing that with the other hand, but at least for me it's not that little muscle memory that has to be retrained.
It's AltGr+7/0 though (on german and swedish layouts at least), which is much worse. Open curly brace is a killer if you are doing it with one hand only. Also the AltGr button is only represented on the right side of the keyboard while you get the Shift on both sides, which makes it more awkward to use with two hands in some situations.
edit: Wow, just learned that Ctrl+Alt is the same as AltGr from the comment below. I've only used computers daily for 21 years ...