Relationships, or attributes, are represented using a rectangle with rounded ends and a double line border, as shown below:
Figure 4.2-1: Attribute element
A rounded rectangle with a single border is permitted in informal contexts (e.g., diagramming during a whiteboard discussion).
"Is A" Arrows
"Is a" (subtype) relationships are highlighted by using an open-headed arrow as shown below:
Figure 4.2-2: "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.