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Span (Cloth)

A span is an old unit of length based on the distance between the tip of the thumb and the tip of the little finger of a fully extended hand. Traditionally, it measures about 9 inches or 22.86 centimeters.

In the context of cloth and textiles, the span was used as a convenient, informal measure to estimate small lengths of fabric before standardized rulers became widespread. It was helpful for tailors and traders to quickly gauge lengths during measuring and cutting.

While the span is largely obsolete as a formal unit today, it reflects historical measurement practices tied closely to the human body, similar to the cubit or hand.

Summary:

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  1 span ≈ 9 inches (22.86 cm)

  
  
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  Based on the hand’s spread

  
  
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  Used historically in cloth measurement and tailoring

  
  
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  Now mostly of historical and cultural interest

Fermi

A fermi is a unit of length used in nuclear physics to measure extremely small distances, such as the size of atomic nuclei. It is equal to 1 femtometer (fm), which is 10⁻¹⁵ meters.

The unit is named after the Italian physicist Enrico Fermi, known for his work in nuclear physics.

Key facts:

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  1 fermi = 1 femtometer = 10⁻¹⁵ meters

  
  
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  Used to measure nuclear dimensions and particle sizes

  
  
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  Essential in the study of nuclear and particle physics

  
  

The fermi is vital for describing distances at the subatomic scale, much smaller than atoms and molecules.