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w:u (Underline)

Applies underline formatting to the run, with a w:val attribute specifying the underline style.

Parent elements

Description

w:u applies underline formatting to the run. It is defined in ECMA-376 Part 1 §17.3.2.40.

The w:val attribute selects the underline style; the optional w:color attribute sets the underline colour independently of the text colour. When w:val="none" the underline is explicitly removed (used to cancel underline inherited from a style).

Attributes

Attribute Type Possible Values Description
w:val ST_Underline single, double, thick, dotted, dottedHeavy, dash, dashedHeavy, dashLong, dashLongHeavy, dotDash, dotDashHeavy, dotDotDash, dotDotDashHeavy, wave, wavyHeavy, wavyDouble, words, none Underline style. words underlines only non-whitespace characters. none removes underline.
w:color ST_HexColorAuto 6-digit hex RGB (e.g. FF0000) or auto Colour of the underline. Defaults to the run text colour when omitted.

Examples

<!-- Single solid underline -->
<w:rPr>
  <w:u w:val="single"/>
</w:rPr>
<!-- Red wavy underline: spell-check or grammar indicator -->
<w:rPr>
  <w:u w:val="wave" w:color="FF0000"/>
</w:rPr>
<!-- Double underline: emphasis -->
<w:rPr>
  <w:u w:val="double" w:color="0000FF"/>
</w:rPr>

Underline Styles

Style Appearance Use Case
single Single solid line Standard underline
double Two parallel lines Emphasis, titles
thick Thick solid line Strong emphasis
dotted Dots Emphasis
dash Dashes Emphasis
dashLong Long dashes Emphasis
dotDash Dot-dash pattern Emphasis
dotDotDash Dot-dot-dash pattern Emphasis
wave Wavy line Spell-check indicator
wavyDouble Double wavy line Emphasis
words Underline non-whitespace only Technical formatting
none No underline Toggle cancellation

Underline Usage Patterns

When to Use Each Style

Style Typical Use Example
single Standard emphasis, hyperlinks Underlined text, Click here
double Strong emphasis, important text Very important notice
thick Maximum emphasis, warnings WARNING: Read carefully
dotted Subtle emphasis, edits Suggested change
dash Emphasis, strikethrough alternative Emphasis without full strike
wave Spell-check indicator misspelled words
words Emphasize content only Skip spaces and punctuation

Spell-Check Convention

<!-- Red wavy underline: common spell-check visual -->
<w:rPr>
  <w:u w:val="wave" w:color="FF0000"/>
</w:rPr>

This is conventionally interpreted by humans as a spelling error even without additional UI cues.

<!-- Standard hyperlink format: blue underline -->
<w:hyperlink w:anchor="_top">
  <w:r>
    <w:rPr>
      <w:rStyle w:val="Hyperlink"/>
      <w:u w:val="single"/>
      <w:color w:val="0563C1"/>
    </w:rPr>
    <w:t>Click here</w:t>
  </w:r>
</w:hyperlink>

Words-Only Underline

<!-- Underline only non-whitespace characters -->
<w:rPr>
  <w:u w:val="words"/>
</w:rPr>

Result: “This text” renders as This text (spaces not underlined)

Useful for:

  • Form fill-in blanks: “Name: _____”
  • Signature lines: “Approved by: _____”
  • Technical documentation: Skip underlines on punctuation

Underline Color Strategies

Match Text Color (Default)

<w:rPr>
  <w:u w:val="single"/>  <!-- Color defaults to text color -->
</w:rPr>

Independent Underline Color

<w:rPr>
  <w:color w:val="000000"/>   <!-- Black text -->
  <w:u w:val="single" w:color="FF0000"/>  <!-- Red underline -->
</w:rPr>

Transparent/Invisible Underline

<w:rPr>
  <w:u w:val="single" w:color="FFFFFF"/>  <!-- White underline (invisible on white background) -->
</w:rPr>

Useful for preserving formatting structure without visual impact.

Form Design Example

<!-- Label + fill-in line -->
<w:r><w:t>Name: </w:t></w:r>
<w:r>
  <w:rPr>
    <w:u w:val="words"/>
  </w:rPr>
  <w:t>____________________________</w:t>
</w:r>

Notes

  • Underline Style: w:val controls line pattern (single, double, dotted, wavy, etc.).
  • Underline Color: w:color is independent of text color; defaults to text color when omitted.
  • Toggle Property: w:u w:val="none" cancels underline inherited from style.
  • Words Only: w:val="words" underlines letters and digits but skips spaces.
  • Color Format: 6-digit hex RGB (e.g., FF0000 for red).
  • Spell-Check Style: Wavy underlines (wave, wavyHeavy, wavyDouble) conventionally indicate spelling/grammar.
  • Hyperlinks: Hyperlinks are typically underlined and colored; adjust these independently if needed.
  • Complex Script: Use w:uCs for Arabic, Hebrew, and complex scripts paired with w:u.
  • Print Display: Underline patterns render as designed in print; some may simplify to single line on legacy printers.
  • Accessibility: Underlines can indicate emphasis or errors; combine with color or text changes for clarity.
  • Form Design: Use words underline for signature/fill-in lines in forms.
  • Compatibility: Standard underlines (single, double) are supported everywhere; fancy styles may vary by Office version.
  • Overuse: Avoid excessive underlines; prefer other emphasis methods (bold, color, style).