This property generalises the notions of "copy of" and "similar to" into a directed relationship, where the domain expresses the derivative, if such a direction can be established. Otherwise, the relationship is symmetric. If the reason for similarity is a sort of derivation process, i.e., that the creator has used or had in mind the form of a particular thing during the creation or production, this process should be explicitly modelled. Moreover it expresses similarity in cases that can be stated between two objects only, without historical knowledge about its reasons.