I recently mentioned the probability that Sir Harry Trelawny, the Sacerdotal Baronet, went to stay in Switzerland with Sir John St Aubyn, his philoprogenitive baronet neighbour from Cornwall. Philoprogenitive? I think, in his case, one might spell the word Phyloprogenitive. He had fifteen children, all of them out of wedlock.
This was the sort of thing the Georgian upper classes did; but not out of a mere mechanical passion for promiscuity. There was a technical reason. Suppose you had a son by an agreeable mistress, and you then decided not to bother to acquire a suitable wife. You now had a strong reason for avoiding marrying the mistress until she had passed the age of child bearing. Because, if you married her now, her next son, being legitimate, would be the automatic heir to your title and to all your entailed property, whereas your eldest, illegitimate, son would get nothing.
If you delay marriage until your lady has stopped having children, you can leave your eldest son, even though he is illegitimate (or even whichever son you may choose) as much of your property as you can rescue from the entail. (The title, of course, will either go to a cousin or become extinct, depending on circumstances.)
Sir John St Aubyn, at St Michael's Mount in Cornwall, and Lord Egremont, at Petworth in Sussex, both did this. As a bastard, you need suffer very little. Any stigma attached to your bastardy rather evaporated, and, after a tactful decade or two, the Crown granted you another baronetcy, or peerage, to compensate for the one you had been unable to inherit.
One group of people, however, did their best to ensure that your bastardy was permanently memorialised ... for all eternity. The hard men and cruel in Queen Victoria Street. The College of Heralds.
The undifferenced coat of arms of a family can only lawfully descend to the legitimate heir, the Head of the Family (don't be taken in by firms which offer you "your Family Coat of Arms" for £20; what they sell you will, almost certainly, at best be the Arms of the Head of some Family with the same surname as yours, to which Arms you are not entitled. Simply because you share a surname with someone, you are no more entitled to use his Arms than you are to live in his house, to eviscerate his bank account, or to sleep with his wife. Or he with yours ...)
So, as a bastard-made-good, off you went to the College to get your armorial situation regularised. And what the Kings of Arms granted you would be the Family Arms differenced, that is, with a change to the Arms to show that you are not the legitimate Head of the Family. And what they granted to bastards was very commonly the Family Arms within a Bordure Wavy.
So you can look out for this wavy border at Petworth or St Michael's Mount ... and probably in quite a number of other places.
I have a fantasy.
In these stately homes, each room is equipped with a Volunteer to keep an eye on the silver and to answer questions. Nice, retired, middle-class people, they are usually well primed with regard to Chippendale Commodes, but a bit vague about Heraldry.
In my fantasy, I will spot an example of some such differenced arms on some artefact and then go up to one of these volunteers and adopt the persona and traditionally-expected manner of an American Tourist. In my best and broadest "Boss Hogg" accent, and in tones loud enough to be heard in all the neighbouring rooms, I will observe "GEE, see here, a BORDURE WAVY! That means he was A REAL BASTARD!!"
I have not so far done this because of apprehensions about Matrimonial Disapproval.