<P> In Merise, Elmasri & Navathe and others there is a preference for same - side for roles and both minimum and maximum cardinalities . Recent researchers (Feinerer, Dullea et al .) have shown that this is more coherent when applied to n - ary relationships of order greater than 2 . </P> <P> In Dullea et al. one reads "A' look across' notation such as used in the UML does not effectively represent the semantics of participation constraints imposed on relationships where the degree is higher than binary ." </P> <P> In Feinerer it says "Problems arise if we operate under the look - across semantics as used for UML associations . Hartmann investigates this situation and shows how and why different transformations fail ." (Although the "reduction" mentioned is spurious as the two diagrams 3.4 and 3.5 are in fact the same) and also "As we will see on the next few pages, the look - across interpretation introduces several difficulties that prevent the extension of simple mechanisms from binary to n - ary associations ." </P> <P> Chen's notation for entity--relationship modeling uses rectangles to represent entity sets, and diamonds to represent relationships appropriate for first - class objects: they can have attributes and relationships of their own . If an entity set participates in a relationship set, they are connected with a line . </P>

Distinguish between an entity set and an entity instance