^Right. So I think Roddenberry's intention was probably that Kirk commanded the "destroyer-equivalent" vessel as a commander and the Enterprise, a heavy cruiser, as a captain. What we should've been seeing all these years were commanders in charge of smaller ships like the Grissom and maybe even the Reliant (which has often been classed as a frigate by fan blueprinters). But I guess the producers either didn't know the naval practice or thought it would be too confusing to have officers of commander rank addressed as "captain." DS9 was the only show that ever did that, with Dax when she commanded the Defiant.