Let us say at once that it is usually left open, but this immediately leads to the question: “Why?”.
There are several explanations related to tradition, but the one that seems to us the most amusing is that which indicates the king of England, Edward VII (b. 1841 – d. 1910) as the main reason. Well, this sovereign had a prominent belly, so he used to keep closed only the first button of his jacket; the King’s subjects, not wanting to seem disrespectful, in his presence began to keep the last button open and this, over time, became part of the court etiquette. A habit that has evidently spread to the present day, even outside of England. Apart from the legends, there is also a practical reason: keeping the last button open, the jacket tends to fall better on the body avoiding that in some points a certain unattractive tension is created in the fabric.
And in the case of three-button jackets? In this case, the gentleman will keep only the central button closed ????
However now you can see jackets with one single button more and more frequently: a good way to resolve doubts in a radical way.