I’ve been using Markdown since 2006 and for me it’s always been:

*emphasis* (i.e. italic)

**strong emphasis** (i.e. bold)

You can use underscores in exactly the same way, but the reason I’ve stuck purely to asterisks is that it’s just slightly easier to type and I think it’s slightly more readable when I’m reading an un-rendered Markdown document, so I appreciate the syntax that John Gruber chose here.

> Given that underlining is how you italics when you can't italics

In my experience underlining is how you bold when you can't bold.

In my experience, underscores are how you underline when you can't underline. Asterisks are emphasis, which is typically italics.

> are how you underline when you can't underline

Underscores/underline were a hint on a manuscript to the typesetter that you wanted italic (https://en.wikipedia.org/wiki/Underscore). It later became a way to add emphasis (either italic or bold styling) on typewriters as there wasn't a practical way to have bold and italic versions of every character on a mechanical typewriter.

Most professionally typeset content (i.e., optimised for legibility through years of experience rather than just whatever your word processor will allow) will not contain underlined content (https://practicaltypography.com/underlining.html).

That's completely true, but emphasis is normally rendered by italics too. (See for example the <em> element in HTML with the default CSS.) Emphasis is the semantic, italics is the presentation. From this perspective both underscores and single asterisks should render into italics.

> emphasis is normally rendered by italics too

I wasn't disagreeing with that; edited my comment to be clear. Asterisks are emphasis; emphasis is typically italics.

I do think that underscores should have been underlining, but unfortunately that's not what Markdown chose.

FWIW, https://style.mla.org/underscore-instead-of-italics/

> In handwritten and typewritten material—where italics are impossible to render—titles of works normally italicized, words used as words, and letters used as letters are underlined:

What in the world does "words used as words" mean?

When you are for example referring to some word or phrase instead of trying to use it in its normal place in the grammar of a sentence. Mean can have many meanings. You may be more familiar using quotes for that.

https://www.chicagomanualofstyle.org/qanda/data/faq/topics/N...

Ah, thank you.

“What in the world does _words_used_as_words_ mean?” would be a valid way of formatting that question according to the guide: that's what they mean.

> “What in the world does _words_used_as_words_ mean?” would be a valid way of formatting that question according to the guide: that's what they mean.

I think you are answering my question, but I still don't understand. How else than as words would words be used? Are you saying that the underlining here is serving the purpose of quotation marks?

CAPS is how you bold when you can't bold.

For decades (centuries?) in German, s p a c i n g was how you “bolded” (or italicized) when your typesetter couldn’t. Very elegant solution once your eye adjusts to it.

Funny of you to say that so authoritatively when you're only demonstrating my point: there is no one correct way to replace bold text when you can't use bold text.

Underlining always looks like hyperlinks now.