Historically, in the early years of the suit as everyday menswear, it appears there were no formal buttoning rules. Look to trade magazines and illustrations from the earlier part of the 20th century, and one sees jackets with between one and five buttons, each buttoned in a manner that suited the personality of the wearer or the cut of the garment.
But much of this changed with a king who was too fat to button his jacket. Or at least, that’s what legend says.
According to the lore of menswear, in the early 1900s King Edward VII started the trend of leaving the bottom button of a suit undone.
Apparently, he gained weight and he was unable to fasten the bottom button of his waistcoat and jacket. To not offend the king, those associated with him started doing the same. The custom then gradually spread the world round (as England was still largely an imperial power with great influence across the globe).