And this is what they the sages established:cxlviiBT Ḥagigah 15a ‘There is nothing between the higher waters and the lower waters, but a hair's breadth.’ And of this hair it is stated:cxlviiiBT Berakhot 48b, Yoma 38b, Mo’ed Qatan 28a ‘No kingdom touches upon its fellow, even by a hair’s breadth’. And with it: ‘the blessed Holy One is exact with the righteous, even as a thread of hair’.cxlixSee BT Yevamot 121b and Baba Qamma 50a And this thread is the thread that encompasses the sign of the covenant of circumcision. And the covenant is a ‘hair-strand’.