Following the Dartmoor "tin rush", Plympton was designated in 1328 one of four Devon stannary towns, (so called from the Latin word for tin, stannum). The other three were Tavistock, Ashburton and Chagford. Tinners were required to take their metal to a stannary town, where the tin was weighed and stamped, and duty paid on it. In Plympton, this took place in Fore Street at a weekly tin court. Twice a year officials checked the quality of the tin at a "coinage". From each block of pure tin, weighing two to three hundred pounds, a small corner, or quoin, was chiselled for testing. Our word "coin" comes from this process.
Stannary courts rigorously upheld the stannary laws. Tin was obtained from Dartmoor by the process known as "streaming" where, as in gold panning, the metal is separated from gravel by using water. As a result of washing through these deposits, many of the streams draining Dartmoor became full of silt. Richard Strode, Plympton's Member of Parliament, introduced a bill in 1512 to curb tin mining on the Moor. He said that the streaming was responsible for the rapid silting of the Plym and other estuaries. He was tried by courts in all four stannary towns, convicted of acting against the interests of the tinners, and imprisoned in Lydford Castle. On his release Strode returned to Parliament, and a statute was passed granting him and other Members immunity from legal action. This was the beginning of Parliamentary Privilege, a cornerstone of our democracy.