A relation is a mathematical term for a two-dimensional table such as the one shown in figure 3.15. It is characterised by rows and columns, each entry there being a data item value. The reason for calling this a relation rather than a matrix lies in the lack of homogeneity in its entries — the entries are homogeneous in the columns but not in the rows. A relational data base consists of such relations, which can be stored on a physical device in a variety of ways.