A chain is a unit of length used in some older systems of measurement, particularly in surveying and land measurement. In the United States, one chain is equal to 66 feet or 22 yards. Internationally, it's defined as exactly 20.1168 meters.
Chains were historically used for measuring large plots of land and for surveying purposes, often in agriculture and real estate. However, the use of the chain as a standard unit of measurement has largely been replaced by the more modern and standardized metric and imperial systems, which use units like meters, feet, and yards.
The term "chain" comes from the practice of physically measuring land using a chain made of linked metal bars or wire. Each chain was typically 66 feet long, and surveyors would lay out chains end to end to measure distances across large areas. Today, modern surveying equipment such as electronic distance measuring devices (EDMs) and GPS technology have largely replaced the use of physical chains in land surveying.
A "pole" is a unit of length that is equivalent to 16.5 feet or 5.0292 meters. It is sometimes referred to as a "rod" or a "perch," and these terms are often used interchangeably. Poles, like rods and perches, were historically used in some English-speaking countries, primarily in the United Kingdom and the United States, for land measurement, surveying, and real estate.
While the use of poles, rods, and perches has become less common in modern times, you may still encounter these units of length in certain contexts, especially in older land descriptions or surveys. In everyday measurements and official standards, more commonly used units like meters or feet are preferred.