Moreover, the use of Cartesian unit vectors such as x ^ ^ , y ^ ^ , z ^ ^ as a basis in which to represent a vector is not mandated. Vectors can also be expressed in terms of an arbitrary basis, including the unit vectors of a cylindrical coordinate system ( ρ ρ ^ ^ , ϕ ϕ ^ ^ , z ^ ^ ) or spherical coordinate system ( r ^ ^ , θ θ ^ ^ , ϕ ϕ ^ ^ ). The latter two choices are more convenient for solving problems which possess cylindrical or spherical symmetry, respectively.