Underlining in Markua using Leanpub's plain text editor writing mode is governed by a setting.

To use underscores to underline text, go to your book's Generation Settings page here:

...making sure to replace YOUR_BOOK with your book's unique web address.

You can also find the "...." page on your book's "Overview" page:

Scroll down until you see the "Italicize Underlines" section:

Click to turn the "Italicize Underlines" feature off:

Scroll down and click the "Update Generation Settings" button:

The next time you create a preview, if you have used underscores like this:

...you will see the part in between the underscores is underlined:

The reason we do underlines this way is a combination of technical and historical reasons.

First, underlining on typewriters was actually a way for authors to indicate to publishers that you wanted the underlined text to appear in italics, believe it or not.

(It would have made no sense to have a completely different set of physical typewriter keys that were shaped in italics, so some markup indicating "make this italics" was required.)

Second, as we explain here:

"In Markdown as defined by John Gruber, and in CommonMark and GFM, *one asterisk* and _one underscore_ both produce italics, and there is no way to produce an underline in Markdown except using inline HTML."

So, using underscores to underline things is governed by this setting on Leanpub, and by default, underscores are used to do italics.

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