When we say in analytic geometry that a point has co-ordinates (x, y), the order in which x and y occur, in the symbol (x, y), is important: (1, 2) ≠ (2, 1). For this reason we call (x, y) an ordered pair. Moreover, x and y come from sets; in this case x, y ∈ R. This idea can be generalizedf as follows. Let 𝒰 be a universe. We can then form ordered pairs (w, v) where u, v ∈ 𝒰 two such ordered pairs (u, v), (x, y) are equal iff u = x and v = y. It is therefore important to distinguish between the ordered pair {x, y}, and the unordered pair {x, y} consisting of the set whose elements are x and y (so {x, y} = {y, x}. Also we can, without confusion, call x the first co-ordinate and jy the second co-ordinate of (x, y).