|(Source: The Sartorialist)|
It's not universally understood how to button a jacket up. Convention dictates that the last button is never done up and this stems from King Edward VII. As he put on weight he unbuttoned the last button on his jacket to give him some relief and the gentry followed suit. The convention has stuck probably because it is a less rigid way of wearing a jacket and looks better as well.
Always leave the bottom jacket button unbuttoned to be sartorially correct.