The Alt or Alt Gr Key
On some international keyboards the left and right "Alt" keys are not the same!
Most PC Keyboards have an Alt key on either side of the Space Bar. In many countries, these keys may be used interchangeably.
However in some localities, especially those where accented characters are used, pressing the right-hand Alt Gr key has the same effect as pressing Alt + Ctrl together. The right-hand Alt Gr is used to generate a third character from certain keys.
For example, on the top row of a British keyboard, shifted 4 generates the Dollar Sign: $. If you hold down the Alt Gr key instead of the Shift key and press 4, it produces: €, the Euro Currency Symbol instead.
Incidentally, in English braille, the Euro symbol is dot 4 plus e.