The medieval universities formed as guilds of masters (teachers) and/or students on the model established by the crafts guilds. Once the scholars were able to receive a charter, they would begin negotiations with municipal authorities to secure fair rents for lecture halls and other concessions. Because they had no investment in a physical campus, they could threaten to migrate to another town if their demands were not met. This was not an empty threat. The scholars at the University of Lisbon in Portugal migrated to Coimbra, and then later back to Lisbon in the 14th century. Scholars would also go on strike, leave the host city, and not return for years. This happened at the University of Paris after a riot in 1229 (started by the students). The university did not return to Paris for two years.