 Nonetheless, that B-properties entail A-properties is neither necessary nor sufficient for A-properties to supervene on B-properties. (The notion of property entailment in play is this: property P entails property Q just in case it is metaphysically necessary that anything that possesses P also possesses Q.) To see that such entailments do not suffice for supervenience, consider the properties being a brother and being a sibling. Possessing the former entails possessing the latter; every brother is a sibling. But being a sibling does not supervene on being a brother. Two people can differ with respect to being a sibling despite being exactly alike with respect to being a brother. To see this, suppose that Sarah has a sister and Jack is an only child. Thus Sarah is a sibling and Jack is not, though neither is a brother. So the B-properties can entail the A-properties, even though A does not supervene on B.