Although it is easily visualized on the graph (which is a curve), the notion of critical point of a function must not be confused with the notion of critical point, in some direction, of a curve (see below for a detailed definition). If "g"("x","y") is a differentiable function of two variables, then "g"("x","y") = 0 is the implicit equation of a curve. A critical point of such a curve, for the projection parallel to the "y"-axis (the map ("x", "y") → "x"), is a point of the curve where formula_2. This means that the tangent of the curve is parallel to the "y"-axis, and that, at this point, "g" does not define an implicit function from "x" to "y" (see implicit function theorem). If ("x"0, "y"0) is such a critical point, then "x"0 is the corresponding critical value. Such a critical point is also called a bifurcation point, as, generally, when "x" varies, there are two branches of the curve on a side of "x"0 and zero on the other side.