Key Attributes:

An important constraint on the entities of an entity type is the key or uniqueness constraint on attributes. An entity usually has an attribute whose values are distinct for each individual entity in the entity set. Such an attribute is called a key attribute, and its values can be used to identify each entity uniquely. In diagrammatic notations each key attribute is represented by underlined attribute inside the oval. For example in the following figure name is the key attribute of the company.