The Assizes of Clarendon and Northampton (1166 and 1176) provided that those suspected of serious crime should be presented on oath by twelve men of each hundred to the king's justices, members of the great council and later the judges of the common law courts—who therefore travelled round the country receiving these presentments and, after the abolition of the ordeals in 1215, presiding at trials by jury for serious crime. At first in the General Eyre and later under the commissions of oyer and terminer and gaol delivery, they would hear criminal cases.
When the petty or possessory assizes were instituted by Henry II, the writs which set them in motion called on the sheriff to summon a group of neighbours (the ‘inquest’ or jury) to give an answer under oath, before the royal justices, to a specific question relating to disseisin. The justices were therefore said to ‘take the assizes’; indeed, the barons demanded in Magna Carta that the justices should travel round regularly for this purpose.
So the ‘justices of assize’ travelled round to hear cases of serious crime and at the same time to take the assizes, i.e. receive the verdicts of the inquest jury in the possessory assizes. Increasingly with the growth of royal justice into the common law, and especially after the reign of Edward I, they also in effect heard civil cases under the nisi prius system.
In the 13th cent. the term ‘assize’ came to be the general term applied to the visits of the judges on circuit. After 1340 the justices of assize were required to be justices of the Court of Common Pleas or King's Bench or serjeants at law.
The assizes continued until 1971 on the circuits ordained by Henry II, the assize towns, which were centres of importance in the Middle Ages, being visited periodically and with considerable ceremony by assize judges, who would hear serious criminal and important civil cases. Although the Courts Act 1971 abolished the assizes, senior judges still go ‘on circuit’ to hear cases in important modern centres of population.
"assizes." The Oxford Companion to British History. . Encyclopedia.com. (November 10, 2018). https://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/assizes
"assizes." The Oxford Companion to British History. . Retrieved November 10, 2018 from Encyclopedia.com: https://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/assizes