There are several reasons to care about how and when to button your suit jacket:
It sends a signal that you pay attention to the details.
In most situations, this makes a suit look better on a man. A buttoned suit when standing cuts a cleaner silhouette.
Prevent the buttons from popping. When seated, unbuttoning allows you to sit more comfortable, prevents wrinkles and keeps the buttons from popping.
Suits are tailored with the assumption that the bottom button will never be fastened. Modern suits are cut in such a way that the fabric doesn’t drape properly when the bottom button is fastened for a two button or three button jacket.
Important to note that we’re talking about SUITS here. Sports jackets are usually buttoned the same way but the rules are much more relaxed because it’s a more casual style.
The most common men's suits are a single-breasted suit with one button, two buttons, three buttons, and a double-breasted suit.
Buttoning Rules For Single-Breasted Suit Jackets:
Because of its origins in traditional eveningwear designs, one-button suit jackets are often cut longer than other types of suits.
Keeping the button fastened maintains a balanced proportion. These jackets should ALWAYS be buttoned when standing. Unbutton the jacket when sitting down, so that it doesn’t crease.
The traditional way to button a two-button jacket is to Always fasten the top button and leave the lower undone.
"Optional, always, never” refers to each of the three buttons. When standing, it is optional to button the top, the middle always, and the bottom never.
Buttoning Rules For Double-Breasted Suit Jackets
Double-breasted jackets are almost always worn buttoned. It’s very unusual to unbutton one.
Button all the buttons that have working buttonholes.
If you plan to leave some buttons undone, it is most traditional to fasten the top button. However, men who prefer a longer line have been leaving the lowest button undone, including members of the British royal family, so you’re probably safe either way.