The entailment "was a subject on which Mrs. Bennet was beyond the reach of reason, and she continued to rail bitterly against the cruelty of settling an estate away from a family of five daughters, in favour of a man whom nobody cared anything about."
This research section looks at some of what's happening in The Heir and The Enchantress. While a couple of plot points are alluded to, there aren't really any spoilers if you've read the other books in The Enchantresses series. What we'll cover here: entailment, settlements, and fee tails. Don't worry--no legal jargon involved! This will be informal. An important point to establish right away is that we're dealing with the property entailment of landed gentry, not aristocracy, which is notably different. More on that to come.
Let's open with a fact we know from the other books in the series: Walter, Hazel's son, is set to inherit the Trethow's estate. That estate is entailed to him. This fact was mentioned in The Earl and The Enchantress and in The Baron and The Enchantress, as well. The sticking point is how an estate can be entailed to the owner's sister's son. Doesn't an entailment work through primogeniture?
The intention behind an entailment, generally speaking, was to ensure the estate stayed within the family. This was important if you, as the head of the household, realized your eldest son was a bit of a wastrel and likely to sell off the land and family home at the first chance he could get. Or let's say you had only daughters but you didn't want their husbands to get their greedy paws on the crib. How about if you knew your eldest son and his son were at odds, and you worried he might try to disinherit that son? Or even let's suppose you remarried and were devoted to your second family and estranged from your children from the first marriage so wanted to ensure the children of your second wife inherited the estate instead of those of the first. There are any number of reasons why you might legally ensure the estate stayed within the family or went to a certain person or line. That is, at face value, the intention of the entailment.
Two points of clarification: (1) when entailing property, the estate owner could decide to whom and which line the property would be entailed. There wasn't a one size fits all entailment. Typically, yes, an entailment was chosen to follow primogeniture--inheritance by the eldest son of the direct line--but that was the most common choice rather than the rule. If the estate owner wanted to entail the property to his second cousin half removed, he could. (2) An entailment could be broken or barred if the current owner and the heir of the estate both agreed to sign a deed breaking the entailment. This would return the estate to freehold, allowing it to be inherited by will or family line or even sold.
Now, before you ask, "If that's the case, then why didn't the Earl of Grantham and Matthew Crawley in Downton Abbey break the entailment since neither wanted Crawley to inherit?" know that the entailment of titles (and the estate that goes with the title) is different from the entailment of estates by landed gentry. An entailment, which is a strict settlement, enables the current owner to name the inheritor or line of inheritance, as well as any stipulations--for example, this property is entailed by primogeniture unless my heir is not married by his 40th birthday. Since the entailment of titles and estates is created by the Crown, the inheritance follows the inheritance line set by the Crown, and should ought go awry, such as no known heirs, then the property and title revert back to the Crown. The properties and titles are owned by the Crown with the titled individual "holding" the title and estate for his lifetime. The entailment is set by the Crown, and thus it follows (in most cases) primogeniture. On the other hand, the properties of landed gentry are owned by the person, and thus an estate entailment is personal and created through solicitors to follow the inheritance line set by that property owner. There's a short bit about this in the Nobility research section where I compare the situation in Downton Abbey to the situation of the Bennets and Mr. Collins in Pride and Prejudice. (Scroll down to the entailment paragraphs.) This article from the Jane Austen Society of North America is a must read when it comes to understanding entails as seen in Pride and Prejudice.
As a landowner, the legal holding is a fee simple in that the ownership of the property is freehold and passes by way of inheritance to the "heirs of the body." When entailing a property, there were three basic choices: fee tail male, fee tail female, and fee tail special. The fee tail male followed the male heir (and the heirs of his body), the fee tail female followed the female heir (and the heirs of her body), and the fee tail special followed specific conditions which could name the heir to inherit (and the heirs of their body), conditions to be met prior to inheritance, and even exclusion of certain people and lines.
What we're typically dealing with in historical romance is an aristocrat who is trapped in an entailment he cannot break because fee tails by the Crown cannot be broken, or an entailment by a family member who wouldn't break it for the world (i.e. Mr. Collins in P&P), or some variation thereof. We're usually reading about an entailment that has already been set rather than an entailment being created. Given the flexibility of entails for landed gentry, there is a plethora of stories in the making, especially with the fee tail special and whatever conditions might be set in place.
Now here's something to ponder which is more in relation to the earlier books of The Enchantresses series than to The Heir and The Enchantress. If Walter, to whom the estate was entailed, and Mr. Cuthbert Trethow, the current owner of the estate, both agreed that the entailment was a silly idea, why did they not agree to sign the deed to break the entailment? The current owner and the heir could do this relatively easily. This question might give new insight into the characters, eh? It makes you wonder which of them refused to break the entail. My money is on Mr. Trethow. Perhaps he kept the entail as a way to (a) encourage his daughters to marry, (b) encourage his eldest daughter to marry Walter, (c) secure peace of mind that Walter would take care of his daughters should neither marry--which tells us that rather than be proud of his eldest daughter's independence, he regretted encouraging said independence. Food for thought (anyone up for rereading The Earl and The Enchantress to dig further into that?).
A great starter source to really capture the heart of the strict settlement can be found here. This source digs into the nitty gritty details of choosing who to inherit and even discusses the breaking of an entailment. As mentioned, the most common type of strict settlement was the fee tail male, wherein the eldest son of the direct line inherited. The estate owner when creating the settlement could just as easily create a fee tail female or even a fee tail special, which is what we see happen in The Heir and The Enchantress, an event that will have a direct impact on the plot in The Gentleman and The Enchantress. No spoilers here since we already got a sneak preview in the epilogue of The Colonel and The Enchantress, but in Hazel's story, we see her father create a fee tail special that entails the property to the heirs of Hazel's body on the condition of his son not having sons of his own.
For some myth busting of commonly believed property ownership and entailment falsehoods, check out this page. It's a quick and educational read. For not so quick reads but detailed and educational, especially if you really want to research settlements further, check out these two articles: this article focusing on Pride and Prejudice entailment and this article offering a thorough examination of the strict settlement.