I believe the guideline is to capitalize the first and all essential words in a letter's greeting. Dear Mom and Dad, Dear Parents, Dear Students, Dear Members, Dear Teachers, Dear Production Manager, Dear Employees, and so on. A letter to a huge group should also be addressed as "Dear All." There are exceptions, though; someone writing a formal letter to a small group of people might write: "Dear Mr. Johnson and Ms. King" or "Dear Members of the Board." Capitalization is also used for foreign governments. Essential words are those that give information about the letter's content without needing further clarification from the reader. If you're writing to tell someone that they've been awarded some prize or honor, then you would include their name in your letter. You could also include the name of someone important to the recipient, such as their employer or professor. However, if you were writing to tell someone that they had been fired, it wouldn't be necessary to include their name. They already know who it is that you're writing to them about. Students are considered essential words because they give information about the recipient of the letter.