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How to underline type in Adobe Illustrator
Putting an underline on type in Adobe Illustrator is an option of the Character palette. To get this palette, go to Window>Type>Character. Then select Show options as shown below.
From there, select the icon of the T with the underline as shown below
It's hidden away kind of for a reason, and I tend to agree. Underlining is a poor way to show emphasis with type, it is much better to use bold, or color, or italics. And especially as underlining is used for hyperlinks on the web, it ends up looking like it should be a hyperlink. I recommend avoiding underlining, but if you must, well, there it is.