Old hands from the early days of DOS and Windows will remember entering special characters by holding down the ALT key and typing a 3-digit number on the numerical keypad. This was essential when using the line-drawing (╠╣║) or block-drawing(▐▬▌█) characters. Then Windows got more advanced and began supporting multiple character encodings in unicode format. And the number of ALT-key digits went from three to four, with the leading zero making a difference (for example, ALT-0174 is “?” while ALT-174 is “?”). I’ve never yet figured out the mapping between unicode and the ALT codes (sometimes it’s very straightforward, sometimes not), so I just gave up and learned to use the Windows Character Map applet (like this: ?). Or, if coding HTML, I would use the HTML character entities (like this: ©).
The linked site gives ALT codes for common non-typeable characters. It shows me that I can simply type ALT-0169 to get ?. The site presents its lists in a manner amenable to printing and posting on the wall near a computer. It’s a bit of a time saver if you frequently use non-English characters or non-typeable special characters.