Editing text on a computer is pretty simple. You can underline a section of text with the keyboard shortcut Command (on CTRL) + U. However, on your phone, things are a little different. Since you have to deal with so many apps on your phone and no two apps work the same, editing text on these apps can be a bit of a challenge.
iPhone 12: How to Bold/Underline/Italicize/Strikethrough Text in the Notes App
If you want to underline text on your iPhone, this post will let you do so in all popular apps.
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Apps that rely heavily on texts in general also allow you to easily edit the textual content. For example, Apple's Pages, Numbers, Notes, Mail and Reminders apps offer a native way to apply underlines to texts. The same can also be achieved with some third-party apps: Google Docs, Sheets, Gmail, Word and Google Chat.
