• May specify which pairs are in relation using some condition for selection; e.g.

    - second element is 1:

R = {(x, y) | x ĪD1, y ĪD2, and y = 1}

- first element is always twice the second

S = {(x, y) | x ĪD1, y ĪD2, and x = 2y}
  • Consider three sets D1, D2, D3 with Cartesian Product D1 X D2 X D3;