I have no idea what the accepted practise is, but it really bugs me when a URL uses an underscore and is then underlined as a link and you can’t see the underscore any more. But that is probably an HTML thing rather than a wider typographical problem.

I’m sure there are people here who can be more useful than me (I’d quite like to know myself).

Ed.

accepted parctice is to match underline and underscore.
but Ed’s point is a good one. I guess that since you don’t have spaces in urls, then you must assume that there’s an underscore there.

For those poor souls who work in Word or whatever, aligned underlines and underscores make for neater-appearing forms...for what that’s worth.

Here’s my approach: The underscore glyph is designed to work when it replaces a space in word_strings_like_this. The underscore’s thickness can matches the horizonal stroke in glyphs like “f” and “t”. (You want it to look good, so don’t match a really fat font with a really fat underscore if it’s goofy.)
The underline should similarly work and look good. I make all underline settings in a font family the same, quite thin (30 units), and position them approximately in the middle of the descender depth. Since underline is either off or on, you want to make sure it looks good running through a word like “jugpoppy” with its many flavours of descenders.
Mes deux sous.

Here I go picking up a very old thread, but it would be interesting to hear more opinions on this. I have looked at a whole lot of fonts. It seems that there are three different approaches here, well actually more, but if we divide them in three approaches, they could look like this:

1. Underline = underscore = same positions and width in the whole family. Light, bold, regular – they have the same underscore/line value.

2. Underline = underscore = same positions and width, but they both change together, depending on weight. Is it light, bold or regular; then the underscore and underline also shows it. It “follows” the other characters, so to speak.

3. All the others: These ones don’t really follow any logic. Underline have one position and one weight and is placed below the underscore – or in the middle of underscore – or alignes on the top of the unnderscore. The underscore could change in weight, but not the underline – it always stays the same.

What could be the ultimate solution to this? I think I prefer alternative two, but maybe there is some crucial information about this today?

An underscore is a distinct character with special meanings in various scripting/programming/markup languages, not the least of which is HTML (principally in frame and window targeting, in that instance). In those situations (i.e.: writing code), at least, it would be prudent to have an underscore character that can be easily recognized as such even when underlined, and not be mistaken for an underlined space.

An underscore is a distinct character with special meanings in various scripting/programming/markup languages

But are underlining really used when programming code? Is is even possible?

As far as I know, no, underlining is not used when programming code. However, I could conceive of it being used in textbooks teaching programming, and there an underlined underscore might come into play.