The bakers of London are first found in the *Pipe Rolls of 1155, when they made an annual payment to the *excheq-uer. The bakers were given authority by London's aldermen to enforce the assize of bread within two miles of the city, excluding the city of Westminster. Although short weight was an obvious failing, this might be evaded by adding sand to flour - something bakers themselves might easily discover, as would the buyer's mouth. Those who infringed were dragged through the city's streets, the offending loaf hanging from the neck. In time, a white, refined bread became popular; this caused a split in the 14c between white-and brown-bread makers. All loaves were required to carry the baker's mark, known as a *'dock', as smiths were required to mark their wares.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.