But what if there isn’t an eldest son?
If a title-holder has no legitimate children, or has only daughters, then the lot goes to the nearest male relative of the title-holder. The next in line would be the title-holder’s next-younger brother, and then his sons (if he has any). If the younger brother has no children or only daughters, the title descends to the next brother in the original family, and then to his sons.
An eldest son is known as the heir apparent, because no matter what happens, if he outlives his father he will inherit. Because no one can come between him and the title, he is the apparent – obvious – heir.
If there is no oldest son, then whoever stands next in line is known as the heir presumptive. Since the title-holder could still sire a son (no matter how unlikely that might be), the heir presumptive could still be pushed out of the line of succession. So he’s presumed – but not guaranteed – to be the heir.
If the title-holder dies without a surviving son, but his widow is pregnant, then everything comes to a halt until the baby is born. If it’s a boy, he will hold the title from the moment of his birth. But if it’s a girl, then the next heir in the male line wins the jackpot.
I used this scenario in my Regency novel, Gentleman in Waiting – where the entire family is gathered, waiting to see whether Lady Abingdon’s child will be a boy or a girl...