This may not be the perfect place to ask this question, but I think it'll work.

Even since I've had my first legitimate suit, I've known that you are not supposed to fasten neither the bottom button of your blazer nor your waistcoast, if you have one, reasons being both that one King of England was simply too fat to do so and it became the fashionable thing to do, and because, simply, your waistcoat will ride up when you sit down. Of course, rules also state that you are supposed to undo all of you blazer's buttons before you sit, lest you ruin it.

However, in several images I have seen recently, both of gentlemen in the early 20th century and of advertisements for tailors and their clothes, all buttons are done up; including the last.

So . . . what is it? I am very curious; it seems that this rule is a more recent thing than I thought.

Thanks!