There are many kinds of relationship that occur in everyday life. Some of these
describe how the members of a family are related to each other: parent, child,
brother, sister, sibling. We could also have a relation called is in for cities and
countries: for example, London is in Great Britain, and Paris is in France. Or
we could have a relation that describes which make of car is produced by which
manufacturer. Relations are used in mathematics to describe how two numbers
are related to each other; for example expressions like x