Similar to concept type hierarchy, circuits are forbidden. The only difference with concept type hierarchy is due to signatures. The constraints are:

1) relation types are grouped by arity. Each 'arity family' must have a maximal element. It means that the relation type hierarchy is decomposed w.r.t. the arity and a unique maximal element is required for each of these sub-hierarchies.

2) Let A and B be two relation types in the same sub-hierarchy (i.e. A and B have the same arity). If A is a kind of B, it means that every concept type in A signature is respectively a kind of concept type in B signature. For example if graze(herbivore,plant) is kind of eat(animal,food) their signatures respect compatibility if herbivore is a kind of animal and if plant is a kind of food.

 graze(herbivore,plant) is kind of eat(animal,food)

herbivore is a kind of animal and plant is a kind of food


Another way to quickly define or pre-define signatures is to use 'Suggest signature...' command on popup menu. Use the command after a link is established between a new relation type and an immediate greater relation type: the parent signature is automatically proposed. But this command is designed for more a complex purpose.In a complex ontology it becomes difficult to define a new relation type signature. The command 'Suggest signature...' can help to find the maximal compatible signature.

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