Page "Tuple relational calculus" Paragraph 2

We first assume the existence of a set C of column names, examples of which are " name ", " author ", " address " et cetera.

We define headers as finite subsets of C. A relational database schema is defined as a tuple S = ( D, R, h ) where D is the domain of atomic values ( see relational model for more on the notions of domain and atomic value ), R is a finite set of relation names, and