Lunar calendar
Lunar calendars were not only methods for measuring time; they indicated both the phases of the moon and a woman’s menstrual cycle. Lunar markings found on prehistoric bone fragments show how early women tracked their cycles and planned families in correlation with the phases of the moon. The earliest form of counting "artifacts are menstrual calendar bones, notched with correct lunar cycles, tabulating pregnancy and menstruation." The prehistoric calendar bones include the Ishango bone, the most well known, which was discovered on the shore of a lake in Zaire, Africa, and represented a six-month lunar calendar. The Isturitz Baton represented both a five- and four-month lunar calendar. The Blanchard Bone Plaque represented a two-month lunar calendar.