66 CHAPTER 4 Types of rules This is also true of relationships: each person was born in a particular place, and that place was at that time in a particular country. 1 4.1.2 Single-valued and multi-valued attributes and relationships Some attributes of physical world objects are single-valued, in that no instance of that type of object has more than one value of that attribute at the same time. For example, each person has only one date of birth. By contrast, while some cars are of only one color, a car may be multi-colored. Similarly with relationships: a person or thing cannot be in more than one place at the one time. In particular, a person can only be born in one country; in contrast, people may be simultaneously citizens of more than one country. 2 4.1.3 Variant and invariant attributes and relationships Some attributes of physical world objects are invariant, in that each instance of that type of object has a value for that attribute that does not change. For example, each person has only one date of birth, which does not change. By contrast, although each person has only one weight at any given time, that weight (unfortunately!) changes over time. Similarly with relationships: a person's country of birth does not change 3 although his or her coun-