Solidarity is unity (as of a group or class) that produces or is based on unities of interests, objectives, standards, and sympathies. It refers to the ties in a society that bind people together as one. The term is generally employed in sociology and the other social sciences as well as in philosophy or in Catholic social teaching. What forms the basis of solidarity varies between societies. In simple societies it may be mainly based around kinship and shared values. In more complex societies there are various theories as to what contributes to a sense of social solidarity. Solidarity is also one of six principles of the Charter of Fundamental Rights of the European Union and December 20 of each year is International Human Solidarity Day recognized as an international observance.