AltGr (also Alt Graph) is a modifier key found on many computer keyboards (rather than a second Alt key found on US keyboards). It is primarily used to type characters that are not widely used in the territory where sold, such as foreign currency symbols, typographic marks and accented letters. On a typical, Windows-compatible PC keyboard, the AltGr key, when present, takes the place of the right-hand Alt key: if not engraved as such, that key may still be remapped to behave as though it is, (or emulated using a chord such as Ctrl+Alt). In macOS, the Option key has functions similar to the AltGr key.
AltGr is used similarly to the Shift key: it is held down while another key is struck in order to obtain a character other than the one that the latter normally produces. AltGr and Shift can also sometimes be combined to obtain yet another character. For example, on the US-International keyboard layout, the C key can be used to insert four different characters:
C → c (lower case — first level)
⇧ Shift+C → C (upper case — second level)
AltGr+C → © (copyright sign — third level)
AltGr+⇧ Shift+C → ¢ (cent sign — fourth level)