The role of citizen is detained has the data of each new citizen with the exact time, when the oath of citizenship was made. The requirements for citizenship varied by city and time period. Decisive was a good repute and proof of income in the form of real property, or the exercise of a trade. Often the recording was connected with the payment of a civil money, and then it was a privileged citizen of the inhabitants of the city.
The University of Cologne offers the volumes of 1875 to 1913 as digitized.