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Chain (US Survey)

The US survey chain is a unit of length used historically in land surveying in the United States. It is defined as exactly 66 US survey feet, which is slightly longer than the international foot. Since one US survey foot equals about 0.3048006096 meters, the US survey chain is approximately 20.11684 meters long.

Like the standard chain, it is divided into 100 links, each measuring 0.66 US survey feet. The US survey chain was widely used in the Public Land Survey System to measure land parcels and boundaries.

Key facts:

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  1 US survey chain = 66 US survey feet

  
  
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  1 US survey chain ≈ 20.11684 meters

  
  
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  10 US survey chains = 1 furlong

  
  
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  80 US survey chains = 1 US survey mile

  
  

Though the US survey foot and chain are being phased out in favor of international units, the US survey chain still appears in historical land records and some legal contexts.

Mil

A mil is a unit of length commonly used in engineering and manufacturing to measure very small distances, especially thicknesses. It is equal to one-thousandth of an inch (0.001 inch), which is approximately 0.0254 millimeters.

The mil is widely used in industries like plastic film, coatings, wire diameter, and mechanical parts, where precise measurements of thin materials are necessary. It should not be confused with the millimeter, which is 0.001 meters or 1,000 microns — much larger than a mil.

Key points:

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  1 mil = 0.001 inch = 0.0254 mm

  
  
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  Used mainly for small thicknesses or diameters

  
  
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  Common in engineering, manufacturing, and material sciences

  
  
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  Not to be confused with millimeter (mm)

  
  

The mil remains an important unit in technical fields where fine measurement is critical.