What is a chain used as measuring unit?

The chain unit for measuring land is equal to 66 feet and a fourth of a  chain is equal to one rod unit, which is 16 1/2 feet. In the world of a surveyor a chain unit was actually made of linked metal chain. Each link unit measures seven and a half inches and there are 100 links in a chain.

Eighty chains equal 5,280 feet, which is one statute mile. Used as a measure of area there are ten square chains in an acre. In the past, many city streets were 66 feet wide because the surveyors found it convenient to use their chain measures in planning them. Nowadays, the old metal chain is replaced with a flexible metal tape   66 feet long.