In the world of men’s shirts, button-down and button-up cannot be used interchangeably (though they often are). So what’s the difference between a button-down and button-up shirt? Simple.
A button-up shirt refers to any shirt that buttons up the front.
A button-down shirt refers to the collar of a button-up shirt. If the collar can be fastened to the shirt via buttons, then it’s a button-down shirt. (See photo above.)