Custom and Convention
As the word "convention" indicates, what is conventional is in the realm of the voluntary. The institutions that human beings voluntarily institute are the products of nurture, not nature. That which is natural is the same everywhere, but positive laws and customs are conventional. As the Greek sophists said, fire buns here and in Persia, but the laws and custom of the Persians and the Greeks are different.
On many aspects of human conduct the laws enacted and enforced by those who have the authority or power to do so are silent. Where the laws are silent in a community, customs that have existed from time immemorial have the force of law. In the development of the English common law, for example, what is established by ancient customs prevails.