A community pledge in medieval England whereby all men over 12, within a defined group, for example 10 households, but sometimes much larger, were jointly held responsible for the denunciation of any crime within their group.
Instituted by the Normans after their conquest of the British island, the frankpledge left community members responsible for each other's conduct, requiring that they investigate and denounce any member of their group who committed a crime or even a disturbance of the peace.
At varying intervals, usually annually, frankpledge hearings were convened.
Some frankpledge records survive like 1411 case of William Collier, who had apparently attacked a certain Thomas, with a dagger, causing him to bleed. William was fined and his dagger confiscated. A frankpledge (that William would pay the fine) was given by his fellow villager, John Plott.
The Frankpledge was the precusor of the present-day American criminal law institution of the Grand Jury.
References and further reading:
University of Leicester, "Life in Silverstone in the 15th Century", published at le.ac.uk/elh/whittlewood/silverstone.htm