Further, time was recorded through the initiation by circumcision. Each initiation group was given a special name. According to Professor Godfrey Mũriũki, the individual initiation sets are then grouped into a regiment every nine calendar years. Before a regiment or army was set, there was a period in which no initiation of boys took place. This period lasted a total of four and a half calendar years (nine seasons in Gĩkũyũ land, each season referred to as "imera") and is referred to as "mũhingo", with initiation taking place at the start of the fifth year and going on annually for the next nine calendar years. This was the system adopted in Metumi Murang'a. The regiment or army sets also get special names, some of which seem to have ended up as popular male names. In Gaki Nyeri the system was inversed with initiation taking place annually for four calendar years, which would be followed by a period of nine calendar years in which no initiation of boys took place ("mũhingo"). Girls, on the other hand, were initiated every year. Several regiments then make up a ruling generation. It was estimated that ruling generations lasted an average of 35 years. The names of the initiation and regiment sets vary within Gĩkũyũ land. The ruling generations are however uniform and provide very important chronological data. On top of that, the initiation sets were a way of documenting events within the Gĩkũyũ nation, so, for example, were the occurrence of smallpox and syphilis recorded. Girls’ initiation sets were also accorded special names, although there has been little research in this area. Mũriũki only unearths three sets, whose names are, "Rũharo" "Kibiri/Ndũrĩrĩ" [1895, "Kagica" "Ndutu/Nuthi" [1897. All these names are taken from Metumi (Mũrang’a) and Kabete Kĩambu.