Arbelos

In geometry, an arbelos is a plane region bounded by a semicircle of diameter 1, connected to semicircles of diameters r and (1 − r), all oriented the same way and sharing a common baseline. Archimedes is believed to be the first mathematician to study its mathematical properties, as it appears in propositions four through eight of his Book of Lemmas. Arbelos literally means "shoemaker's knife" in Greek; it resembles the blade of a knife used by cobblers from antiquity to the current day.