Relations describe relationships between some or all elements of one or more sets. The exceedingly simple definition of a relation in Sect. 4.1 should not deceive; any kind of relationship and binding can be formally captured with it. In daily life, we encounter such relationships, e.g., between people (Hans is friends with Klaus, Katja is the daughter of Sibylle and Kurt) or between objects and properties (the pillow is soft, the heating is warm).