After 701, the nengō system was always used, even including the present day. Before 701, nengō were not used during the gap years between Hakuchi and Shuch. Also, nengō were not used in another gap between Shuchō and Taihō.
↑NengoCalc (645) 大化 Taika, online conversion of Japanese dates into their Western equivalents; calculation is based on tables from Paul Yachita Tsuchihashi. (1952).
Japanese Chronological Tables from 601 to 1872 A. D. (邦暦西暦対照表) and
Reinhard Zöllner (2003), Japanische Zeitrechnung; retrieved 2012-11-14.
↑ 17.017.1Brown, p. 270; excerpt, "The eras that fell in this reign were: (1) the remaining seven years of Shuchō [(686+7=692?)]; and (2) Taika, which was four years long [695-698]. (The first year of this era was kinoto-hitsuji .) ...In the third year of the Taka era , Empress Jitō yielded the throne to the Crown Prince."
Genealogic tree for the Emperors of Japan, showing the Daikakuji-to and Jimyoin-to branches (in Spanish)
Table of the Emperors of the Northern Court and the Southern Court during the Nanobuku-chō period (in Spanish)
Looking forward: Maybe there something about the process of simplifying this pre-Taihō period which might suggest ways to better explain the nengō of the Nanboku-chō period in a visual way? --Ansei (talk) 18:00, 14 November 2012 (UTC)