One-button jacket: classic tuxedo jackets feature one button, which is to be buttoned.
Two-button jackets: button the top button, leave the bottom undone.
Three-button jackets: this one is just a little more complicated. First, always leave the bottom button undone and button the middle button. Whether to button the top button on a three-button jacket depends on the style of the lapel. In a "3-roll-2" jacket, the roll of the lapel covers the top button, essentially turning the jacket into a two-button jacket.
A 3-roll-2 jacket. |
This jacket may maintain cleaner lines with the top button buttoned. |
Double-breasted jackets: with these, the bottom button is optional, with all other buttons buttoned.
When should your jacket be buttoned? As often as possible, but always while standing. Your jacket was designed to be buttoned and, if properly tailored, is more flattering (i.e., slimming) when buttoned. Some single-breasted jackets will not present as well if buttoned while sitting, though, so you may want to unbutton it to sit. (Double-breasted jackets generally look better buttoned at all times, including while sitting.)
Addendum [8/15]: a wise reader writes to suggest we mention the royal origins behind the bottom-button rule. The Art of Manliness has a good description:
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.For today's purposes, the most important reason to follow these rules is that nearly all modern men's jackets are designed with the intent that the bottom button will remain unbuttoned. Thus, buttoning the bottom button will likely negatively affect the lines and drape of the jacket and may cause unsightly bunching.
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 grew so rotund that 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).