History
Back in the 19th century, suits were designed for all buttons of a suit to be buttoned. However, in the early 1900s, King Edward VII of Great Britain became too large in diameter to comfortably fit in a fully-buttoned suit. Thus, he began to leave the bottom button of his suit jacket and vest unbuttoned. To make the King feel better, his subjects also started to unbutton the bottom buttons of their suits and vests, and this trend eventually spread to the commoners and continues until today.
Ever since the mid-20th century, men’s suits have been designed so that the bottom button serve only as a decorative purpose – meant to be left unbuttoned.
How to Button
Currently, the two-button-single-breasted jacket is the most popular style for suit jackets and sport coats. On a two-button jacket, the top button is fastened, while the bottom is often left undone.
Taller individuals often prefer a three-button jacket, but it requires a more complex rule. The bottom button is never buttoned, the middle one is always buttoned, and the top button is sometimes buttoned. The buttoning of the top button depends on the cut of the suit. To “test” whether your three-button suit demands the buttoning of the top button, fasten the middle button and see whether the top button “closes” on its own. If the suit does, button the top button; if else, button only the middle button
For one-button jackets, such as dinner jackets, the only button of the jacket should be always buttoned. However, there is no rule for single-breasted suits with more than three buttons. Button these jackets as you see fit.
All buttons of a double-breasted suit are traditionally buttoned. However, in recent years, it has become popular to unfasten the bottom row of buttons on a double-breasted suit with more than two rows of buttons. The latter is more suitable for modern double-breasted jackets.
When to Button
British PM David Cameron and US President Barack Obama wear their suits unbuttoned while sitting (AP Photo/Carolyn Kaster)
Appropriate buttons on a single-breasted jacket are fastened when standing and unfastened when sitting. Some who wear bespoke or otherwise well-fitting jackets keep their jackets buttoned while sitting to “show off” its tailoring. Additionally, many choose to button their jackets while on camera, regardless of posture, because a closed jacket often looks better on camera.
By tradition, a double-breasted suit is always fully-buttoned regardless of posture. However, the bottom row of buttons is sometimes unfastened when sitting to relieve stress on the suit’s buttons.
Vests
Vests are similar to suits in buttoning etiquette – the bottom button is always unbuttoned per British tradition. So, technically, European vests could be buttoned fully, but the difference between British and European-cut vests is so little that it is safe to leave all vests’ bottom buttons undone. For more casual occasions, it might also be appropriate to undo the top button of the vest for symmetry.