I think the reasons for those choices are: * It's very uncommon to want your output text actually underlined. * If you really do want underlining, just use <u

I think the reasons for those choices are: * It's very uncommon to want your output text actually underlined. * If you really do want underlining, just use <u>this</u>, since it's fine to put raw html in Markdown source. * The most common way folks express emphasis is probably with ✱asterisks✱ (HN won't let me backslash-escape the *). A less-common alternative is to use _underscores_ (though I rarely see that anymore these days). Maybe the distant third most-common way was with /slashes/. I guess Gruber had to draw the line somewhere and just support the first two most common ways to write it. That seems reasonable. Another reason to not use slashes for emphasis: using them might interfere (or at least cause headaches) with writing directory names, fractions 1/2, and "and/or" wordings.