WHY doesn't the bottom button of a jacket get buttoned?
This etiquette has been observed since the beginning of the XX century. Historians assume that the rule of wearing a jacket appeared thanks to the English King Edward VII (1841-1910), who was obese. He used to leave the bottom button undone so that a tight vest would fit better.
As a sign of respect for his king, the British court, and after him all the inhabitants of England and the British colonies, no longer buttoned the bottom vest. This theory is not taken seriously by everyone, and some trendsetters have challenged it with other versions.
Fashion designer Hardy Amies explained the single-breasted suit was first introduced in 1906 and was later called the "leisure suit".
It became a casual garment, had a wide cut, and the three buttons gave the suit a proper fit. Over time, the jacket replaced traditional riding coats. And as the third button was below the waist, it was unbuttoned while sitting on a horse for comfort.