The gender binary is the idea that human gender exists in two forms: masculine and feminine. The term also describes the system in which a society divides people into male and female gender roles, gender identities and attributes. Gender role is one aspect of a gender binary. Every known society has used the gender binary to divide and organize people, though the ways this happen differ among societies. A universal aspect of the gender binaries is that women give birth. Gender binaries exist as a means of bringing order. Certain notable religions are often used as authorities for the justification and description. Islam, for example, teaches that mothers are the primary care givers to their children and Catholics believe only males may serve as priests.