A calculation disc for indication of fertile and infertile days. The method is based on the findings of Hermann Knaus and Kyusaku Ogino from the 1930´s.
Knaus and Ogino were the first in human history, who managed to correctly identify fertile and infertile days. This counting device can be used with 28-days´cycles only. The calculation disc was produced and sold by a Styrian company for each new calendar year.