This is from MS-DOS. The keyboard gives you access to about 100+ characters. However, the codes that define a character range from 0-255. If you want to see what some of these extra “letters” look like:
Hold down Alt
Type in a 3 digit number; 0-255
Let to go of Alt
The character will appear
This doesn’t work on every program, but most and particularly the older ones that remember the DOS days. This is a list of the ASCII characters and codes.