For control purposes the subscriber lines on the switches of the LLF are divided into vertical groups of fifty, being five line units on each of ten switches. Each vertical group is divided into five vertical files of ten lines, important because class of service, or customer group identification in later Centrex offices, is shared by all ten lines in the vertical file. Staff in Centrex offices spent much time standing on ladders, rewiring the Class of Service data fields at the top of LLF.