We learned this as appraisers, since some older...

...legal descriptions are still written in the style of metes and bounds with distances in chains. This comes down to us from the practice of surveyors in merrye olde England using actual metal-link chains to measure courses for property delineation. By the 17th century this was all standardized, with 320 rods to the mile, 4 rods to the chain, 66 feet to the chain, etc.

There are interesting vestiges of the old system. Most rights of way in the city of Chicago are 66 feet (one chain or 4 rods) in width for the major streets and 33 feet (2 rods) for the side streets. Alleys are generally 16 feet or about 1 rod.