Comment by Sniffnoy

Professional typesetters have an aversion to underlining text. It's ugly. The line interferes with descending letters: f, g, j, p, q, y. The underline is as thick as the stroke of a character, so decorative marks compete with signal marks. It's just too strong. That's why you see text underlined in homemade flyers but almost never in books, magazines, newspapers, or other things designed by pros. The recent addition to CSS to specify that underlines go behind the descenders is a welcome change.

Professional web designers have the same aversion that print designers have, they are birds of a feather. But now they have another reason to hate it. Underlining has come to mean hyperlink. Because of their original aversion, they will often use CSS to remove that default underline, maybe bringing it back only when you hover over a link. But because it is still a widespread convention throughout the web, and probably always will be, they would never, ever emphasize something by underlining (they use border-bottom ;).

Since you should underline something only if it is a link, which has its own markdown, and since underlining is ugly anyway, and since bold and italics are enough ammunition for all your emphasizing needs, and since it is an established convention from the old days that underlining meant eventual italics when it went to press, it's okay in my book to repurpose _underscores_ for italics.

Slashes instead on both sides like /this/ --- hmm, it's not bad. I'm undecided.

> Most word processors have underline as separate from underline.

One of those words isn't supposed to be 'underline', right?