How does this work?
Using combining diacritical marks.
Diacritical marks are added to letters to describe how they should be pronounced. Accents like the one above é are diacritical marks.
There are hundreds of thousands of possible combinations of letters and diacritical marks. So, Unicode—the standardized system of computer characters—allows us to combine diacritical marks with any character.
Before adding diacritical marks:
A z ! & Ɯ Ԫ ט ጨ ⿔ ♞ 👌
After adding diacritical marks:
A͛ z͍ !͆ &͜ Ɯͫͫ Ԫ̳̳̳ ט̌ ጨ̊̊̊̊̊̊ ⿔͡ ♞́ 👌̸̸̸̸͇͇͇͇͇ͤͤ̏̏̏͡
As you can see, this system is easily abused—and that's what this tool does.
By adding a “Double Macron Below”—which looks like an underscore—to every character, we can emulate an underline.
This may seem like a hack, but it remains true to the roots of underlining.
Underlining is another dreary typewriter habit. Typewriters had no bold or italic styling. So the only way to emphasize text was to back up the carriage and type underscores beneath the text. It was a workaround for shortcomings in typewriter technology.
Matthew Butterick — Butterick's Practical Typography (this website is amazing)
Where can I use this tool?
You can use this underlining hack anywhere you wouldn't usually be able to underline text.
The characters included in the tool are also special. They're from the Unicode block of mathematical alphanumeric symbols—they keep their style wherever you use them.
This means the underline tends to stay the same no matter where you paste it. 𝚕̲𝚒̲𝚔̲𝚎̲ ̲𝚝̲𝚑̲𝚒̲𝚜̲.
If you use this trick without these characters (you can do this by clicking the show more tab), you might find it looks s̲o̲m̲e̲t̲h̲i̲n̲g̲ ̲l̲i̲k̲e̲ ̲t̲h̲i̲s̲—not nice.
Social media sites have different ways of rendering diacritical marks. I've done my best to normalize these across all platforms. The double underline doesn't work well on Twitter.
When should I underline?
Underlined text is hard to read and is generally bad practice. Even the convention of underlining links is disappearing.
The only time it's permisable to underline text is when you're being ironically retro (or writing an article about underlined text).
Otherwise, use one of this websites many other stylised text generators.