Complex sentence: Since 1234, Zürich had been governed by an aristocratic council, composed to one third by members of the nobility and to two thirds by the city's patriciate, mainly consisting of influential merchants.
Split into simpler sentences: Since 1234, Zürich had been governed by an aristocratic council. One third of the council's members were representatives of the nobility, and two thirds were drawn from the city's patriciate, consisting mainly of influential merchants.