From mediaeval times business actvity in the City was regulated by Guilds. They were called livery companies because of the distinctive livery they wore for processions. Most lines of business could only be pursued by members of the relevant livery company. In practice, it was the senior members of the livery companies who provided the manpower for the government of the City. In the early Middle Ages, 3 livery companies (wool, cloth and wine) provided three-quarters of all the Aldermen who held office.
The Barbican Estate has two livery companies as its close neighbours: the Barbers and Surgeons, and the Ironmongers. See the Living section for more on them.

Guildhall
The early City of London Rise of the City of London
Dick Whittington and other Lord Mayors
Livery companies