A surveying chain is used to measure distances. When surveying, it's important that the thing used to measure be held level and tight (ie, not sagging in the middle), so that the measurement is true. It must also be dimensionally stable; not stretch when pulled or expand or contract with humidity or temperature.
In historic times, a real chain was used (40 links, for a total of 33 feet). Later, a much lighter weight steel chain was developed and used up until the advent of electronic distance meters in the late 1970's.
Using a surveying chain is always a two-person job. The rear chainman holds the chain over the rear point, using a plumb bob to extend the point upwards if necessary to keep the chain level. Since only the zero end of the chain (the "small" end) has tenths and hundredths of a foot marked on it (after the zero point), the head chainman holds the zero end within a foot of the forward mark. The rear chainman finds the nearest foot and holds it, while the front chainman pulls the chain taut. When they're both good, the rear chainman calls off the feet and the front chainman calls off the tenths and hundredths of a foot that hang off the zero end. Surveying is quasi-metric: feet are used, but not inches or miles. Instead measurements are in feet and tenths and hundredths of a foot.
In days past, when very accurate surveys were attempted using a chain, surveyors used published tables that would correct for the slight expansion or contraction of the chain at different temperatures. Amazingly, accuracies of 1 part in 50,000 were possible when good technique was used (that is, an error of less than a tenth of a foot over a distance of a mile).
For measuring long distances with a chain, markers called chaining pins are used. These are lightweight metal pins that can be pushed into the ground with one hand. The rear chainman holds the "big end" of the chain over the rear point. The head chainman then stretches the chain tight and marks the zero point with a chaining pin. He then shouts "come ahead" and the rear chainman pulls the rear chaining pin and moves up to repeat the procedure. The distance is easily calculated by counting the number of chaining pins the rear chainman has pulled when they reach their destination (provided, of course, he correctly remembered to start with one in hand to make up for the fact that the first point needed no pin).
Storing a chain at the end of the day involves a cute trick (but one that is hard to visualize). Imagine holding the zero end of a long steel tape (about a quarter of an inch wide and fairly thin and supple) in your left thumb and forefinger. You then extend your right hand back and grab the chain at the four foot mark. Without allowing the chain to twist, you bring it up and place it on top of the zero point. This will leave four feet of chain hanging down with a once-around twist in it (try it with a belt or something, you'll see the twist). You repeat this procedure until all of the chain is hanging down, each loop with a single twist in it. Then, you tie off the end using the leather strap on each end of the chain to bundle the ends together. The loops will hang down in a sort of figure-eight as they twist. You grab the figure eight at the place where it crosses, pull, stretch and bend and pop, the chain will somehow snap into a roll where there are no twists, just a continuous wind of steel in a circle. Learning to "throw" a chain is one of the first skills a chainman is taught. Botching the job of throwing a chain will cause the chainman to become the target of great derision.
See: land surveying.