A key is one or more columns that are identified as such in the description of an index, unique constraint, or a referential constraint. The same column can be part of more than one key. A key composed of more than one column is called a composite key.
A composite key is an ordered set of columns of the same table. The ordering of the columns is not constrained by their ordering within the table. The term value when used with respect to a composite key denotes a composite value. Thus, a rule such as "the value of the foreign key must be equal to the value of the primary key" means that each component of the value of the foreign key must be equal to the corresponding component of the value of the primary key.