Two Alt keys exist on QWERTY keyboards; one is located on either side of the space bar. Both of them exist because it makes it easier to use keyboard shortcuts with the left Alt key or to use the right Alt key with the number pad to create special characters.
Some of the most common keyboard shortcuts use keys located on the left side of the keyboard. For example, Alt + F in many applications brings up the File menu, while Alt + V brings up the View menu. By holding down the right Alt key, typing a four-digit code on the numberpad and releasing Alt, it's possible to type special characters that don't appear on the keyboard