A Professional Greeting. Your letter should be addressed to your boss. There's a good chance that this letter will merely end up in a file in HR, but it starts with your boss. Assuming this is someone with whom you work reasonably close with on a daily basis, it doesn't have to be super stiff and formal. Specific and professional is the right tone here.
However, if you know the right signs to look for, there's a good chance you can figure out what kind of reaction to expect__and that can make the conversation go much more smoothly. Here are a few common reactions and how to anticipate which one you'll receive.