Bernard, you have ASCII code and HTML codes and depending on what you type may give you a different result.

For example ALT 65 & ALT 065 will give you the same result A.
Whereas ALT 189 gives me ¢ and ALT 0189 gives me ½

ALT 65 and ALT 189 is what they call ASCII code

Whereas ALT 065 and ALT 0189 is what they call HTML code.

John in Kansas is the best guy to explain the difference between the two and why you would use the different codes.

However, here is a link to the ASCII code numbers you can use with the ALT key http://www.asciitable.com/
The Dec column is the number you type after the ALT to get the Char column (red characters).

Here is the link for the HTML code http://www.lookuptables.com/
You take only the number you see in the numeric column and put a 0 in front e.g. ALT 0190 gives you ¾

So for all you budding keyboard users get practising :-)