Quick guide"
"HIM" His/Her Imperial Majesty-Emperor or in the past Kaiser of Austro-Hungarian Empire and Tsar of Russia
I believe the Kaiser of the Austro-Hungarian Empire , as both Emperor of Austria and King of Hungary, was referred to as "His Imperial and Royal Apostolic Majesty". Similarly, the German Kaiser, as both German Emperor and King of Prussia, was cited as "His Imperial and Royal Majesty".
Interestingly, the Emperor of Japan (the last reigning emperor in the world) uses the style "His Majesty" without the qualifier "Imperial", even though the princes of the Japanese imperial family are cited as "His Imperial Highness".
In the Austro-Hungarian Empire, the princes of the House of Habsburg-Lorraine were technically "Prince Imperial" of Austria and "Prince Royal" of Bohemia, Hungary and Croatia, but they normally used only the title of "Archduke" with the style "His Imperial and Royal Highness".
Historically, those titles changed over time. In the early 16th century, for example, Kings were normally styled "Highness". The style of "Majesty" was used only by the Holy Roman Emperor. Then, some monarchs like Francis I of France and Henry VIII of England also started using the style of Majesty and, soon, other kings followed their lead.
"HM" His/Her Majesty - King/Queen of a Kingdom: UK, Denmark, Spain etc..
"HRH"-His/Her Royal Highness- Prince/Princess/Royal Duke/Duchess of a Kingdom
"HSH"-His/Her Serene Highness-Principality-Monaco/Lichtenstein.
His Royal Highness is also used by sovereign Grand Dukes and their respective heirs (currently only the GD of Luxembourg). Previously, some sovereign Dukes also occasionally used the style of HRH, or alternatively, only His Highness (HH).
Technically, the children of a sovereign Grand Duke who are not the heir to the throne should use the lower style of His/Her Grand Ducal Highness, but the Princes of Luxembourg currently use HRH instead as dynastic members of the House of Bourbon-Parma.
HRH may be also used by holders of titles of rank equivalent to a royal prince, e.g.
Infantes/Infantas of Spain (children of the Spanish monarch who are not the heir to throne, or children of the Prince or Princess of Asturias), or, in the past,
Enfants de France (children of the King of France including the Dauphin, and children of the Dauphin or of the Dauphin's eldest son) and
Petits-enfants de France (children of a
Fils de France other than the Dauphin or the Dauphin's eldest son).
I would appreciate if Blog Real or some other Portuguese poster could also comment on the princely titles and styles in the defunct Kingdom of Portugal.
I think that of the Grimaldis, the one who is most close to the rest of European royalty is Princess Caroline. I think she had some closeness to Queen Sofia (but I don't know if they are close today) and to the Kings of Sweden.
I think Prince Albert is close to some royals. Princess Stephanie never seemed to be close to members of other royal families.
Princess Caroline and Princess Diana:
https://1.bp.blogspot.com/-gsC-p0pe...640-h429/53a939ec1bb730c05491944e808728e1.jpg
https://1.bp.blogspot.com/-QuwM_2BS...Q/s16000/584f970a7b02dc2181994f8ee8329ded.jpg
https://1.bp.blogspot.com/-eaOLELD6...640-h424/ce1692a0771fb854642d3324158c09cd.jpg
Princess Caroline and Queen Sofia:
https://1.bp.blogspot.com/-Ej5LJgHc...Q/s16000/acee0b0777848a5d348132f5e99afc3a.jpg
https://1.bp.blogspot.com/--jSHdhBK...y-for-a-tuna-fish-reseach-centre-reutersv.jpg
Princess Caroline and King and Queen of Sweden
The King and Queen of Sweden seem to be very close to several former sovereign families of Germany and Austria (judging by the number of events of those families which they attend).
I wonder if they became close to Caroline after she became Princess of Hanover, or if they were already close to her as a Monegasque Princess.