Traditionally an overcoat should be worn as a top layer over your outfit. Not just reserved to keep you warm under a suit - these can be thrown over hoodies for a tradional-with-a-twist winter style. How to fit it? The shoulders of an overcoat should be gently hugging the body, with no hang-time over the line of the arm. This way, if you throw your overcoat over just a thin layer, it won't look boxy or ill-fit.