Originally posted by Gabriel Claramunt:
a collar is associated with a dog or a car
a kennel is associated with a dog and many cats.
Although the UML standard  specifies that multi-valued attributes are allowed and that "" is the default multiplicity of attributes, some authors of popular UML books appear to be unaware of this (e.g. , p. 63). Moreover, some of the principal authors of OMT (the Object Modeling Technique from which UML class diagrams are largely derivative) argue that the default is single-valued with nullability unspecified (i.e. either  or [0..1]).