The parametric equation of lines is a set of functions x(t), y(t) (and if necessary z(t)) that express the points of the line through a parameter t. The vector form r(t) = r0 + v·t, or general r(t) = (x(t), y(t), z(t)). The parameter domain is an interval defining the direction along the line. It is possible to switch to Cartesian coordinates by eliminating the parameter. The condition r'(t) ≠ 0 ensures the quality of the line