Customs are sets of agreed, stipulated or generally accepted standards, norms, social norms or criteria, often taking the form of a custom.
Certain types of rules or customs may become law and regulatory legislation may be introduced to formalise or enforce the convention (e.g. laws which determine which side of the road vehicles must be driven). In a social context, a convention may retain the character of an "unwritten" law of custom (e.g. the manner in which people greet each other, such as by shaking each other's hands).
In physical sciences, numerical values (such as constants, quantities, or scales of measurement) are called conventional if they do not represent a measured property of nature, but originate in a convention, for example an average of many measurements, agreed between the scientists working with these values.