User blog comment:WiKiAN/Community reminder/@comment-169320-20130827170252/@comment-961279-20130828014628

If I remember right, underlining was used back when typewriters only had one font and couldn't do italics. The ones with the arms that moved like a hammer as each key was struck. You could do bold by moving the paper slightly left/right or up/down and typing the same letters again, or on a early computer, you might surround what should be bold with asterisks: *bold*. But the underlining would interfere with any "descenders" like the lower part of g, j, p, q and y.

After that came ones like the IBM Selectric and daisy wheel typewriters where you could change out the type "element", and then you could pause what you were doing, switch to an italic font, then switch back and not fuss with moving the paper.

One place says either underlines or italics can be used, but you pick one and use just that. That link also mentions typewriters' capabilities. But it looks like a style guide from 2002 favors italics and the second guide referenced on that page says that underlines are mostly used now in bibliographies for scholarly works.

Between the two, I prefer italics because it's easier to distinguish from a hyperlink.