Making ASCII chars working under X
 

Hi all, is it possible to make ASCII chars working under X programs, like OpenOffice, but also editing files with VI?
I mean typing ALT + xyz.
After login, and before startx command, this is possible, and it has been said to me that the only way is trying using ALTGR instead ALT button or trying casual ALT or ALTGR letters combinations.
Is it true?
It sounds strange to me because of they works outside X.
Thenks..