In a WPF RichTextBox, the effective style of a Run of text is a result of combining the properties defined on the Run with the properties it "inherits" from the enclosing Paragraph and finally the styles on the Document. So you can set FontWeight to Bold at any of those levels. You can also set it Bold on the Paragraph and then switch it to Normal (override it) for a specific Run.

However, underline and strikethrough are different. They are items that can optionally appear in a list of TextDecorations, which is a property of Inline (and hence Run) and of Paragraph, but not of Document. And you can switch on Underline in the Paragraph, and it gets inherited so that all Runs within that Paragraph default appear underlined by default.

Is it possible to switch it off underline in a specific Run? i.e. is there a way to insert an entry into the list of TextDecorations which would mean "Don't underline", thus overriding the Paragraph's setting?