Off: each word gets its own line.
Underline your text, l̲i̲k̲e̲ ̲t̲h̲i̲s̲, and the line stays when you paste it into a caption, a post or a name. The line is a second character after every letter, and the spaces get one too, so “read this” comes out as one line rather than two. Turn off Line under spaces to give each word its own line. Emoji and line breaks get no line.
Of the chat apps people ask about, Discord is the one with an underline code: two underscores on each side, __like this__. WhatsApp, Slack and Telegram read underscores as italic, and Instagram and TikTok print them as they are. Everywhere but Discord, paste a style from the Underline tab.
Each style is a different mark under or over every letter:
Screen readers stumble on all of them. Every letter carries a mark, and some readers spell the word out, so keep it to a heading or a few words. To take the line off again, paste the text into Fix Fake Text. To cross words out instead, use Strikethrough Text.
Discord underlines anything between two pairs of underscores.
Where an app has no underline button, paste text that carries its own line.
Ctrl+U on Windows, Cmd+U on a Mac. That works in Word, Google Docs, Gmail, Outlook and Telegram Desktop. In a chat app without an underline button, and in Instagram or TikTok, there is no shortcut, so paste Unicode underline instead.
Not with a code. WhatsApp reads _underscores_ as italic, and it has no underline in its format menu. Paste Unicode underline from this page and the line comes along, though the other person sees it in their own font, which can leave small gaps.
Paste it in. Instagram has no underline button or code, but a Unicode underline is part of the text, so it survives in a bio, a caption and a comment. Every underlined letter counts as two characters, so a short headline works better than a whole paragraph.
Mostly for totals. Accountants put a double line under the final figure of a column, after a single line under the subtotal. In Word, Ctrl+Shift+D double-underlines (Cmd+Shift+D on a Mac); anywhere else, copy the Double underline style here.
With <u> for a marked word, such as a misspelling, or CSS text-decoration: underline when you only want the look. Use it sparingly on a web page: readers take an underlined word for a link, click it and nothing happens. For emphasis, <strong> or <em> doesn't have that problem.
Italics, if you can type them. Underlining titles comes from typewriters, which had no italics, so an underline told the typesetter to set the words in italic. MLA and APA now ask for italics; underline is for handwriting.