What are virtically stretched (fence) operators?

MathML defines a list of virtically stretched operators that are stretched vertically to match the maximum height of neighboring elements. Here are some examples:

• Parentheses: ( and )
• Square Brackets: [ and ]
• Angle Brackets: < and >
• Braces: { and }
• Vertical Bars: | and ‖
• Square-Root: √
• Vertical Arrows: ↑, ↓ and ↕

The example below shows that 2 vertical bars are stretched to match the height of the fraction element.

<math><mo>|</mo><mo>&#x2212;</mo>
  <mfrac><mn>1</mn><mn>2</mn></mfrac><mo>|</mo>
  <mo>=</mo><mn>0.5</mn></math>
    $|-\frac{1}{2}|=0.5$

Note that some old browsers do not support automatical stretched operators. In this case, you can add fence="true" and stretchy="true" attributes to force it:

<math><mo fence="true" stretchy="true">|</mo><mo>−</mo>
  <mfrac><mn>1</mn><mn>2</mn></mfrac><mo fence="true" stretchy="true">|</mo>
  <mo>=</mo><mn>0.5</mn></math>
    $|-\frac{1}{2}|=0.5$

Or try to use the <mfenced> element to replace stretched operators:

<math><mfenced open="|" close="|"><mrow><mo>−</mo>
  <mfrac><mn>1</mn><mn>2</mn></mfrac></mrow></mfenced>
  <mo>=</mo><mn>0.25</mn></math>
    $\left|-\frac{1}{2}\right|=0.25$

Or try to add <mrow> next to the operator to trigger the stretchy detection.

<math><mrow><mo>|</mo><mo>−</mo>
  <mfrac><mn>1</mn><mn>2</mn></mfrac><mo>|</mo></mrow>
  <mo>=</mo><mn>0.5</mn></math>
    $|-\frac{1}{2}|=0.5$

⇒ "mrow" as Virtical Stretch Boundary

⇐ Stretched and Large Operators

⇑ Stretched and Large Operators

⇑⇑ MathML (Mathematical Markup Language)