The foremost shrine of each old province
Under the ritsuryo system Japan was divided into some sixty provinces. Each had one shrine regarded as its foremost, its ichinomiya. Because provincial borders differ from today’s prefectures, a single prefecture may contain several ichinomiya.
Who decided? The governor’s round of visits
The prevailing view traces the ichinomiya to the order in which a newly posted provincial governor visited the shrines of his province. On arrival, a governor’s first duty was to visit the principal shrines and present offerings. Differences in how those shrines were treated hardened into a ranking.
No document defines the criteria, however, and several explanations compete: popular devotion, or a rite by which royal authority renewed its covenant with the province’s foremost deity.
How it differs from ninomiya and sannomiya
Shrines ranked below were called ninomiya and sannomiya, understood as reflecting political and social relations within the province. The pattern varied: Noto province had only a first and second shrine, while Nagato treated its first and second as equals.