Thus from the middle of the 1100s A.D., the weavers enjoyed the monopoly of their craft, rights of supervision which ensured a high standard of workmanship, power to punish infractions of their privileges, and full control of their members. In this they stand as the prototype of English medieval guilds. These rights represented the standard which all bodies of craftsmen desired to attain. The right of independent jurisdiction was exceptional. On the north side of the city was a great forest with fields and wells where students and other young men from the city took walks in the fresh evening air. Vendors on the river bank sold cooked fish caught from the river and wine from ships and wine cellars. London's chief magistrate was the port-reeve, who was appointed by the King, until 1191. Then the port-reeve was replaced by a mayor, who was elected yearly by the city wards. Each ward was headed by an alderman and there were city sheriffs and councilors. The mayors were typically rich merchant princes. There were three ways to become a citizen of London: being the son of a citizen, apprenticeship in a craft for seven years, and purchase of citizenship. London growth led to its replacing Winchester as the capital. Over its history, it generally chose or elected its own mayor every year. (This was not a popular election.) But there were many periods when royal authority was asserted over it. St. Barthomew hospital was established in London for the care of sick pilgrims traveling to the shrine of Becket in Canterbury. Trading was facilitated by the stabilization of the amount of silver metallic content of the English coinage, which was called "sterling" [strong] silver. The compass assisted the navigation of ships and London became a major trading center for foreign goods from many lands. About 5% of the knights were literate. Wealthy men sent their sons to school in monasteries to prepare them for a livelihood in a profession or in trade or to the town of Oxford, whose individual teachers had attracted disciples for a long time. These schools grew up around St. Mary's Church, but had not been started by the church as there was no cathedral school in Oxford. Oxford had started as a burh and had a royal residence and many tradesmen. It was given its basic charter in 1155 by the King. This confirmed to it all the customs, laws and liberties [rights] as those enjoyed by London. If became a model charter for other towns.