The number one purpose of the craft guild is to protect the economic interests of its members.
Rules regulate membership and trade.
In the early days, no artisan could work in a town unless he was a member of the local guild. No goods could be imported into a town if they competed with local products. It was in this way, that the local guilds held a monopoly of the market within its own town. Competition amongst members of the same guild was discouraged and the guild would do almost anything to prevent it. Each guild laid down detailed regulations governing the quality of its product, the methods of manufacture, and the price that could be charged for it.