With a double-breasted men’s suit, the jacket is normally kept fully buttoned all of the time.
When you are wearing a single-breasted men’s suit, you don’t need to have all the buttons buttoned up.
Whatever configuration of buttons you have, you should unbutton the jacket completely when you sit.
A single-breasted jacket with a single button should generally be fastened at all times when you are standing.
On a suit with two buttons, the top button only should be fastened – the unfastened button helps the suit drape properly to frame the body in an elegant manner.
With three buttons, the middle button should always be fastened when standing; the top button can also be fastened if that’s comfortable for you and suits the look. The button at the bottom should never be fastened.
If you have four buttons on a single-breasted men’s suit, the middle two buttons should be fastened when you’re standing – fastening the top button as well can make you look a bit over-wrapped, since the button is quite high up the jacket. The bottom button should always be unfastened to help the shape of the jacket.