Plaid shirts can sometimes be tricky, but the right tartan on a casual shirt gives your off-duty look a rakish, seasonal charm. And sporting a plaid shirt at the office with a timely wool tie separates you from the other guys wearing standard solid button-downs. Pair these easygoing styles with your everyday denim, roll up the sleeves and pull on a puffer vest or dress them up to inject some life into a dark wool suit or a tweed blazer.