This is more for my own reference, but if anyone else finds this useful, you’re free to like/reblog it and what-not. Most of the information was either taken from various Wikipedia pages or WikiHow.
First, let’s look at the social hierarchy:
Grand Duke/Grand Duchess
Grand Prince/Grand Princess
When meeting royalty for the first time, always acknowledge them with a bow from the neck (not the waist) if you are a man, and a small curtsey if you are a woman. (This gesture is no longer applicable in today’s world, but if you’re writing for an earlier time period, then it’s important your character bow or curtsey).
Below is directly applicable to citizens of the U.K and Commonwealth:
Only shake the queen’s hand if she offers it to you first. If you are wearing gloves, do not remove them.
Do not begin a conversation with the queen. Instead, wait until she starts speaking to you.
When addressing royalty, finish your first reply with their formal address. For example, if a prince asks you, “How are you enjoying the United Kingdom?” you would respond “It’s wonderful, Your Royal Highness.” Each title carries a different formal address:
Emperors and Empresses are addressed as “Your Imperial Majesty” and introduced as “His/Her Imperial Majesty”.
Queens and kings are addressed as “Your Majesty.” Introduce them as “Her Majesty the Queen” (not "Queen of England", as she is the “Queen of the United Kingdom”, “Queen of Canada” and a long array of additional titles).
Princes and princesses are referred to “Your Royal Highness.” Introduce them as “His Royal Highness the Prince of Wales.” Any child or male line grandchild of a monarch is considered a prince or princess. The spouse of a prince is also a princess, although she is not always “Princess” Her First Name. The spouse of a princess is not always a prince. Great-grandchildren in the male line of the monarch are not considered princes or princesses. Use the courtesy titles lord or lady for these personages, addressing them as, for example, “Lady Jane” and introducing them as “Lady Jane Windsor” (unless they have a different title of their own).
Dukes and Duchesses are called “Your Grace” or “Duke/Duchess.” Introduce the duke to someone else as “His Grace the Duke of Norfolk,” the duchess as “Her Grace the Duchess of Norfolk”.
Baronets and knights, if male, are addressed as “Sir Bryan” (if his name is Bryan Thwaites) and his wife is “Lady Thwaites”. You would introduce him using his full name, “Sir Bryan Thwaites,” and his wife as “Lady Thwaites.”
Dames (the equivalent of knighthood for women - there is no female equivalent of baronetcy) are “Dame Gertrude” in conversation, and you would introduce her as “Dame Gertrude Mellon.”
Other forms of nobility (including Marquess/Marchioness, Earl/Countess, Viscount/Viscountess, Baron/Baroness) are generally addressed as, “Lord or Lady Trowbridge” (for the Earl of Trowbridge), and introduced with their appropriate title, such as “Viscount Sweet” or “Baroness Rivendell” .
Use “Sir” or “Ma'am” thereafter. If the noble uses a casual style of conversation, drop the “Sir” or “Ma'am.” Don’t make them have to ask.
This information strictly deals with meeting British peers and royalty.Other parts of the world have different systems of aristocracy, and while the British royal family’s official website notes that when meeting a member of the royal family, “There are no obligatory codes of behaviour - simply courtesy,” this is not the case for all aristocracies. Failure to observe specific codes of behaviour in some countries may result in harsh punishment.
So—- it’s always best to research the monarchy in which you are writing for. I used the U.K’s peerage system because it’s the most widely known, but don’t take it as applicable for every monarchy. Titles and protocols can differ greatly between cultures.
If any of this information is incorrect, please feel free to correct it.