Here's something that I've wondered about for some time. What is considered the appropriate kind of shirt to wear in casual situations (without a tie) under a nice sports jacket? Is a dress shirt, that is usually worn with a tie and having a stiff collar, appropriate? Or should we not use a dress shirt in such a situation and instead look for a truly "casual" shirt that would never be worn with a tie (less stiff collar, etc.)? A turtleneck works well under a jacket, but I'm concerned here with button-up shirts.