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Button the middle button when standing. Button the top button in some situations. Undo all the buttons when seated. Avoid buttoning the lower button.
The middle button is between the top and bottom button. If your lapel's roll is soft, and extends past the first button, use the middle button. You should always keep this button fastened when standing. It is never customary to undo the middle button when you're standing up. With a flat lapel, you should button the top two buttons. The suit will look better this way. You can also use the top button if you personally prefer this look as long as your lapel does not extend past the first button. Again, the middle button should always be fastened when standing. This is also customary when wearing a three button suit. This will help prevent creases. Always remember to re-fasten the buttons when you stand. Under no circumstances should you fasten the lower button. This is never considered customary. Only use the top and middle button on a three button suit. Some three button suits are called 2 1/2 suits. In these suits, the lower button almost always constricts movement when fastened. It is a particularly big faux pas to fasten all three buttons in a 2 1/2 suit, and it will also be very uncomfortable.