 What are relations and why are they so special? Let’s compare three sentences: “John is an American”, “John is taller than Tom”, and “John is between Tom and Mary”. The first sentence has a unary predicate “is an American”, the second sentence a binary predicate “is taller than”, and the third sentence a ternary predicate “is between…and…”. A unary predicate stands for a property or quality, while a binary or ternary predicate stands for a relation. If a first-order logical system has only unary predicates, then we say it is monadic. Otherwise, predicate logic is assumed to have binary or other higher predicates.