A physician is properly addressed, "Algernon Brigham, M. D.," and the salutation is "Dear Dr. Brigham," or "Dear Doctor," if he is an intimate friend. A man having the title of Doctor with any other significance than that of Doctor of Medicine, is usually addressed "Dr. Frederic V. Harlan." A very formal way, however, would be to address such a one,--supposing each of the titles to be his,--as "Professor Frederic V. Harlan, Ph. D." For the letter, the salutations "Dear Professor Harlan" and "Dear Dr. Harlan" are equally correct.
A letter to the President of the United States should be addressed simply with that title and with no further specification of name, whether it be official or social: as, "To the President of the United States, Executive Mansion, Washington, D. C." The salutation should be simply "Sir." The conclusion should be, "I have the honor to remain Your obedient servant." If a social letter it may be addressed either formally or "To the President of the United States, (Christian name and surname), Executive Mansion," etc. The salutation would then be "My dear Mr. President."
The Vice President should be addressed officially in the same form; that is, "To the Vice President, Hon. Chester A. Arthur." He should be saluted, officially, "Mr. Vice President, Sir;" socially, "My dear Mr. Arthur."
In addressing the governor of a state the superscription should be, "To His Excellency, The Governor of Massachusetts, State House, Boston." The salutation should be "Sir," if official, but "Dear Governor Barnard," if social. The conclusion of an official letter should read, "I have the honor to be, Sir, Your Excellency's most obedient servant."
The mayor of a city is addressed, "To His Honor, The Mayor of Chicago." Within, he is saluted officially as "Your Honor," socially as "My dear Mayor Sewall."
Ambassadors of any country, whatever their personal distinction, may be given the title of "Honorable," and their rank placed after the surname. As, "Honorable Whitelaw Reid, Ambassador to the Court of St. James." They may always be addressed as "Your Excellency."
The members of the Cabinet of the President of the United States are always addressed as "Honorable," and the name of their department, or their title added: as, "The Honorable, The Secretary of State." To give the name would be superfluous, as in the case of the President. On formal invitations, however, when the Secretary and his wife are entertaining, the form is, "The Secretary of State and Mrs. Hay request the honor," etc.