Thus from the middle of the 1100s, 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. In Henry II's charter to London, London did not retain its right to appoint its own sheriff and justice given by Henry I. London's chief magistrate was the mayor, who was appointed by the King, until 1191. Then the mayor was elected yearly by the aldermen of the city wards and approved by the king. He was typically a rich prince chosen by the barons and chief merchants of London. The commoners had no voice in his selection, but they could still approve or disapprove of the actions of the city government at ward and folk motes. At certain periods, a king asserted royal power over the selection of mayor and governance of the city. 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 and Westminster growth led to their replacing Winchester as the capital. St. Barthomew infirmary was established in London for the care of sick pilgrims traveling to the shrine of Becket in Canterbury. It had been inspired by a monk who saw a vision of St. Barthomew telling him to build a church and an infirmary. 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, a magnetic lodestone [leading stone] needle mounted on a cork and floated in a bowl of water, assisted the navigation of ships. With it, one could tell the general direction of a ship when the skies were cloudy as well as clear. And one could generally track one's route by using the direction and speed of travel to calculate one's new position. 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 scholars had migrated from Paris and 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. It became a model charter for other towns.