What is a UK nautical league?
A UK nautical league is three UK nautical miles, ie 18,240 feet or 5,559.552 meters, which rounds to 5.56 kilometers. The international nautical league is slightly shorter at 5,556 meters.
Don't confuse it with the land league, which is three statute miles. The nautical league is three nautical miles, about 3.45 statute miles, so it's roughly 15% longer.
Britain's territorial waters once reached one marine league – three nautical miles – from the coast. The limit has been 12 nautical miles, or four leagues, since 1987.
What is a chain?
The chain is a surveying unit of 66 feet, which is exactly 20.1168 meters. It is divided into 100 links of 7.92 inches each, and 80 chains make a mile.
The chain was devised by the English mathematician Edmund Gunter around 1620 and used to survey land. Ten square chains make exactly an acre, which is why surveyors liked it. GPS has replaced it for most work, though US public land surveys are still recorded in chains and links.