Wow....Extra Fine means Extra Fine!
I was given the TWSBI Diamond 580 as a gift and fell in love. The pen had a Fine nib, which again, I love but I am addicted to the finest nib I can fine so I had to have the TWSBI Extra Fine just to see....and it is wonderful! It was easy to change the nibs out, it only took seconds to be writing with a nib fine enough to fill in the smallest of forms in style.