Royal Titles, Peerage and Gentry Facts for Writers:
Royal titles obviously belong to the royal family, exclusively, and their are essentially only two types of titles that exist within it. There is the King/Queen/Monarch, who is the head of the monarchy or the spouse of the head. Below them is the title of Prince/Princess/Prinxe, who are the direct descendants of the King/Queen/Monarch for two generations, meaning only the children and grandchildren get these titles, anyone following that is not actually a Prince/Princess/Prinxe. These are the only royal titles that exist.
Unfortunately, the male title does always outrank the female title, and the gender neutral title does as well. So, a King's bride may be given the title of Queen, but the Queen's husband would actually have to be given a different title, usually Prince. As for same sex relationships, the King's husband would be a Prince and the Queen's wife would be a Princess. King and Monarch are actually of the same rank, so a Monarch's wife would still be Queen, while the husband would be a Prince, and if the spouse was non-binary they'd have to be Prinxe. The title of Regent is tacked onto an existing title, giving them more power, but only temporarily. So, if the King is ill and unable to do his job, you may end up with a Queen Regent, Prince Regent, Princess Regent, Prinxe Regent. This era is called a Regency, and it's to say that the Kingdom is under the rule of another temporarily. Minor's cannot be Regents.
As for other members of the royal family, such as siblings to the King/Queen/Monarch or nieces/nephews/neib, they do not technically get royal titles, they instead usually end up being given peerage titles.
The peerage is a ranking consisting of five ranks, Duke/Duchess/Dux, Marquis/Marchioness/Marchix, Earl or Count/Countess/Countum, Viscount/Viscountess/Viscountum and Baron/Baroness/Baronum. Again, male title outranks the female title, and the gender neutral title is on the same rank as the female title for this one, since these are new terms while Monarch is an old term. The three highest ranking of the peerage have heirs who are automatically entitled to a title, which they may choose, however the children of Viscounts/Viscountesses/Viscountums and Barons/Baronesses/Baronum are not always going to automatically get a title. The eldest may inherit their parents title, but their siblings will at most be referred to as Lord/Lady/Laird unless they are given a different title. This may encourage them to marry up in the hierarchy.
The reason we tend to think of the siblings of the King/Queen/Monarch as being Duke/Duchess/Dux is because it is the highest title they may be granted without it being a royal title. They're still considered part of the royal family, but since they're not part of the direct lineage, they cannot be referred to with a royal title, thus they'll be given the highest title possible in most cases, although they may be given lower titles for various reasons. Their descendants may also be granted any Peerage that is lower than the one their parents have. So the eldest child of a Duke may be given the title of Marchix, and down the sibling order may get lower and lower titles or all of the same titles (rare since that requires a lot of land).
Marrying into the peerage does not guarantee a title, but a Duke's wife may be called a Duchess and so on for the rest of the titles. Since the male titles do outrank the female and gender neutral titles however, their spouses would have to be given a lower title, usually just Lord/Lady/Laird, although generosity may allow for a peerage for that spouse.
Below the peerage is the gentry, which consist of people who are nobility in some form, as in above the common people, but aren't typically considered noblepeople. The gentry includes Baronet/Baronette/Baronetum, Knight, Esquire and Gentlepeople. These people would typically all be referred to as either Sir/Dame/Ser, or sometimes none of these. Their children again, inherit the rank their parents have, but have no entitlement to a rank of their own. These people will also not be referred to as Lord/Lady/Laird, as they do not rank high enough for those honourifics. They're also not typically considered to be debutants. They don't have a coming out to society thing, nor do they attend the fancy parties.
There are other titles, such as Archduke/Archduchess/Archdux and Margrave/Margravine/Margravum, but the first one is actually specific to Austria and the second is specific to the German border, whereas a lot of the ones I've discussed, while specific to Europe and countries colonised by Europe, don't tend to have any one country their from. Archduke/Archduchess/Archdux would be a royal title, but the lowest one, and Margrave/Margravine/Margravum is a military rank around the same as Marquis/Marchioness/Marchix.
If there's any titles I missed or any details I got wrong, let me know! This is just as far as I know, but I could be wrong in some places.
