In ancient times, toraijin (foreigners, particularly from China and Korea, who settled in early Japan and introduced the culture of the Asian continent to the Japanese) who were well versed in Rekido (study of the calendar) or Sando (study of mathematics) were often appointed, but later only members of the Okasuga clan, Iehara clan, Onakatomi clan or Kamo clan were appointed to the post.
古くは暦道・算道に通じた渡来人系の人物が任じられる事が多かったが、後に大春日氏・家原氏・大中臣氏・賀茂氏などが世襲した。