Relationships, or attributes, are represented using a rectangle with rounded ends and a double line boarder, as shown below:
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Figure 15 attribute element A rounded rectangle with a single border is permitted in informal contexts (e.g., diagramming during a whiteboard discussion).
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"Is A" Arrows
"Is a" (subtype) relationships are highlighted by using an open-headed arrow as shown below:
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Figure 16 "Is a" element The rounded rectangle specified in section 4.2 is omitted when representing "Is a" relationships, and the arrow head always points to the parent (super-type) concept.