if it's a two button jacket, button them both.. if it's a three button jacket, leave the bottom one unbuttoned, so when you are sitting the jacket doesnt get ruffled up, then if you are standing up and moving around and conversing, you can leave the bottom one buttoned and open the top one to show the shirt and tie underneath... my grandpa told me that's the way you're suposed to wear jackets and coats when i was younger