HM stand for His/Her Majesty. It is used by the monarch of a country usually a King or Queen.
- If the Monarch is a King and his wife is Queen, she will often use HM as well.
- TM stand for Their Majesties. It refers to two or more royals who use HM.
- If the Monarch is a Queen, since King outranks Queen, her spouse is usually titled Prince or Prince Consort and uses HRH. (Together they are referred to as Her Majesty and His Royal Highness.)
- (Monaco, Liechtenstein, Luxembourg do not refer to their monarchs as His/Her Majesty.)
- Most abdicated monarchs continue to use HM, however some does not (e.g. Netherlands).
HRH stands for His/Her Royal Highness. It is used by most of the European Royal families for members of the royal family.
- TRH stands for Their Royal Highnesses. It is used when you are talking about 2 or more royals with HRHs.
HSH stands for His/Her Serene Highness, It is used by Monaco & Liechtenstein, as well as a number of former Monarchies. HSH is the equivalent of HRH, although HSH is also used to address the monarch, where HRH is not.
- TSH stands for Their Serene Highnesses. It is used when you are talking about 2 or more royals with HSHs.
- Sometimes you will see TT.SS.HH., it also means Their Serene Highnesses.
HH stands for His/Her Highness. This is used only by a few monarchies for a number of different reason, usually to signify a family member who isn't granted the distinction "Royal" (Think children of the 2nd child of the Monarch). (e.g. Denmark, Norway & Netherlands)
HI&RH stands for His/Her Imperial and Royal Highness. It is a combination of two Royals styles HIH & HRH.
- This style is used when someone belongs to two different royal families. One that uses HRH and one that uses HIH.
If you are even unsure of a royal's proper style or how you should address them feel free to ask.