For a function f :S→T the graph of f is the subset {(s,f (s)) | s ∈ S} of the Cartesian product S×T. Note that for each s ∈ S there is a unique t ∈ T such that (s,t) is in the graph; some authors define functions as such subsets of the Cartesian product of the domain and codomain. It is common to refer to or label the graph of real function f :ℝ→ℝ as simply y = f(x), and surfaces in ℝ³ often arise as graphs z = f(x,y).