Times have changed however. Not all names follow the basic guidelines. What if you, or the recipient, have a double name such as Mary Grace or a hyphenated last name. This is where so many questions come from. The "rules" for this can be debated, however here are some examples of what can be done. The only sticking point with these examples, is that not all fonts can be used - however there are numerous options available so that won't be an issue.
Remember though, any monogrammed item you buy is custom - it can be done how YOU want it. If the item is a gift, and you just aren't sure what the middle initial is then use the first initial. A single initial monogram can be very creative.