Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The coordinate surfaces of a three dimensional coordinate system are the surfaces on which a particular coordinate of the system is constant, while the coordinate lines are the curves along which two of the coordinates of the system are constant. The coordinate lines are often associated with the coordinate which is not constant.Similarly, the coordinate surfaces of the colatitude coordinate are cones, and those of the azimuthal angle are half-planes as shown in the figure on the right.Thus, the coordinate lines for the r coordinate of the spherical coordinate system are the rays that point outwards radially from the origin: such a line is the intersection of coordinate surfaces for (a cone) and (a half-plane) The coordinate unit vectors are the unit-vector normals perpendicular to the coordinate surfaces. They point in the direction of the fastest increase of the corresponding coordinate.