A title is a form of address appropriate to a particular person.
- A person without a title is addressed as "my lord" or "my lady"
- A Lady as Lady
- A Lord as Lord
- A Knight is addressed as Sir
- A Count or Countess as Your Excellency
- A Duke or Duchess as Your Grace
- A Prince or Princess as Your Highness
- A King or Queen as Your Majesty
Note that these are the conventional English versions. Many people prefer titles more appropriate to their persona.
Note also that medieval English kings were addressed as "Your Grace". "Your Majesty" is found in letters to King Henry VIII after about the time he broke with the Church of Rome (watch the gradual transition from "your grace" to "your highness" and "your majesty" in the letters addressed to Henry by his wives and daughters at http://englishhistory.net/tudor/letters.html). If you have examples of the use of "Your Majesty" before this time, please cite them here.