Courtesy titles are the bane of all writers dealing with the aristocracy. First and foremost, if you are one of those who cannot keep it straight, I would suggest this link https://www.chinet.com/~laura/html/titles05.html as it contains a good summary, along with the exceptions.
For those of you I have already lost, a courtesy title is a title that does not have legal significance but rather is used through custom or courtesy, particularly, in the context of nobility, the titles used by children of members of the nobility. A courtesy title is used for children, former wives and other close relatives of a peer, as well as certain officials such as some judges and members of the Scottish gentry. This manner of styling a person are used “by courtesy” in the sense that person referred to by these titles do not themselves hold substantive titles. The British peerage system uses a variety of courtesy titles.
For those who write in the JAFF (Jane Austen fan fiction) venue, we consistently find authors who do not understand how Lady Catherine de Bourgh is not addressed as Lady de Bourgh. From the story in Pride and Prejudice, we know Lady Catherine is the daughter of an earl, for her brother (Colonel Fitzwilliam’s father) is an earl. Most JAFF writers refer to Lady Catherine’s brother as the Earl of Matlock because such is what he is called in the 1995 TV serialized version of Pride and Prejudice, but Austen never presents the character this title.
On a side note, it is likely Miss Austen was modeling the family on that of the real-life William Fitzwilliam, the fourth Earl Fitzwilliam, and one of the richest men in the UK. His splendid estate, Wentworth-Woodhouse, was likely to have been one of the models for Pemberley. William Fitzwilliam was known as a kind and just man. He was an influential member of the House of Lords and served for a short time as the Lord Lieutenant of Ireland where he tried, unsuccessfully, to enact an early form of Catholic emancipation.
Now, back to the issue at hand. As the daughter of an earl, Lady Catherine can be styled as Lady + first name. Her husband was a baronet. We know this because he is addressed as “sir.” With Lady Catherine’s obvious demand for rank and one keeping to one’s sphere and polluting the shades of Pemberley, etc., she would not wish to be known by the lesser rank of Lady de Bourgh and the wife of a baronet. After all, a baronet is a “commoner,” not part of the aristocracy. The daughter of an earl is addressed by Lady + first name + last name. The actual rule is: Daughters of dukes outrank all ladies except duchesses, marchionesses, and royalty. Therefore, unless a duke’s daughter marries an actual peer or another duke’s eldest son, she would be able to maintain the courtesy title derived from her father, and be known as “Lady <Firstname> <Husband’sSurname>” or “Lady <Firstname><Husband’sTitle>” (if he is not an actual peer but has a courtesy title). Thus, Lady Catherine, the daughter of an earl, may, in society, be addressed as Lady + Catherine + (husband’s last name) de Bourgh. She is never Lady de Bourgh (although such is certainly her name); yet, the higher title, the one of being the daughter of an earl is chosen to keep her influence in society.
What really messes with some JAFF writers’ minds is “Lady Lucas.” The Lucases are friends of the Bennets. Sir William Lucas is a “sir” because he has been knighted at St James for his service as mayor to the Meryton (We assume it is Meryton, but Austen never tells the reader this.) Having the title of “knight” is not a title which can be inherited by the man’s children. They are more frequently conferred, compared to the grades of the peerage.
The wife of a knight may use the courtesy title of “Lady” before her surname, provided she uses her husband’s surname. For example, the wife of Sir John Smith is Lady Smith.
Now to a number of questions people have sent me of late.
Question #1: I have a grandson of a marquess in my story. He is the younger son of the heir to the marquess. At the time of my story, the marquess has been dead for some years; his eldest son (this character’s father) predeceased him, and the title passed to the older son of the heir, this character’s half brother. In this situation, would the younger son take a courtesy title (Lord Firstname?) I know he would have if the title had passed in the normal way, father to son to son, but his father would have only had the courtesy use of a lesser title of his father, probably an earldom, at his death. I hope this is not too convoluted! I am thinking he wouldn’t be Lord Firstname in this case, but I just was not certain.
Response: Debrett’s peerage has a section on siblings of peers who have special grants of precedency. In all of these cases, a sibling inherited because the father died before the grandfather—before the father himself could inherit.
When the eldest brother succeeds to the peerage, he can request that his siblings be granted the precedency they would have had if the father had inherited. Though it is called a grant of precedency it really gives the person all the privileges of such rank such as courtesy titles. So, yes, if the younger brother is a half brother on the father’s side, he would have the title of Lord First name.
Valentine Heywood’s book, British Titles: The Use and Misuse of Titles of Peers and Commoners, with Some Historical Notes (1951), tell us when a nephew, a cousin, or other relative not in the direct line succeeds to a peerage, it is customary for the Sovereign to accord the new peer’s brothers and sisters the style which would have been theirs had their father held the title. There is no certainty about it but, other things being equal, it is the practice generally adopted. It is conferred by a Royal Warrant.
Heywood mentions only ones not in the direct line, but it is often a case a marquess’s son is an earl, and the earl’s oldest son is a viscountcy, but the siblings are plain honourables. If the father dies and the viscount becomes an earl, the brothers remain the same but the sisters can be upped to Lady First name, however, all the siblings generally would be raised to the higher ranks when the brother succeeds as marquess. The children’s mother’s rank and precedence doesn’t change.
Question #2: Once a person had a rank, he/she did not lose it. Correct? Example: Peer has daughters, so they are Lady first name. No sons. So when he dies and the title goes to some cousin, a young woman would not be a Lady anything if she were only a cousin to a peer. She keeps her title because she was a daughter to a peer, even if he has passed away, right?
Response: Correct. Just because a father died and a cousin inherited, the children of the deceased do not lose their titles. In the mindset of the day, why would anyone not want to show the birth rank of the bride, if he himself does not have a peerage title?
A duchess who married a tutor when widowed, kept the title of duchess, though such was not strictly legal and was not her legal name at that time. However, socially and in all dealings except with the royal and legal courts, she was a duchess. A man who proposed to the daughter of an earl, marquess, or duke knew what he was getting. It could be a source of pride for the man to show off his wife’s status. Moreover, such was her legal name.
Question #3: I have a character who is being elevated from Earl to Marquess [Marquis, Marquess, etc.], who has an unmarried sister with a dowager countess mother. Now under the earl’s title her address would be Lady Anne. If he is elevated, would that leave the mother still a countess under the earl title? And would the sister, now the only female related to the new title be Lady last name, or still Lady Anne?
Response: Although it is true the sister’s status would not be affected by his elevation, a daughter of an earl and a daughter of a marquis are addressed the same, as Lady First name, with the surname added, depending on the situation. The only way her name would change would be through her marriage to a peer, in which case she becomes Lady Husband’s peerage name.
For most of Western history, marriage was a private contract between two families. Until the 16th-century, Christian churches accepted the validity of a marriage on the basis of a couple’s declarations. If two people claimed that they had exchanged marital vows—even without witnesses—the Catholic Church accepted that they were validly married. During the Regency, couples who chose to marry could be joined by several means. Many chose to purchase what was known as an ordinary, common or standard license. Others chose to have what was known as a calling of the banns. 
“There were two kinds of marriage licences that could be issued: the usual was known as a common licence and named one or two parishes where the wedding could take place, within the jurisdiction of the person who issued the license. The clergyman who administered the ceremony had to be associated with the Church of England. He would issue the licence as a cost of a few shillings to a pound, depending upon the wealth of those applying. The license was valid for 15 days. The couple had the option of marrying in either the parish of the bride or that of the groom. They must be a resident of the parish in which they were to marry. (Do you recall Wickham had to wait to marry to Lydia Bennet until he could establish residency. “We were married, you know, at St. Clement’s, because Wickham’s lodgings were in that parish. And it was settled that we should all be there by eleven o’clock.”) Both the groom and the bride had to make sworn statements that no impediments existed that would cancel their marriage vows.
The poor often married without benefit of clergy or ceremony–they just called themselves married and that was that (made divorce a lot easier and possible, too). Needless to say, the Church of England heartily disapproved. The old custom–marriage by cohabitation had been legal before the Hardwick Act of 1753, and there was the even older custom of handfasting–often held with the very poor, particularly since marriage was more about a legal agreement as to the disposition of property and inheritance from a legal alliance. 
Every Woman Dreams... 