of the form (planes parallel to the -plane) produce plane sections that are often called contour lines in application areas. Mathematical examples of cross sections and plane sections A cross section of a polyhedron is a polygon. The conic sections – circles, ellipses, parabolas, and hyperbolas – are plane sections of a cone with the cutting planes at various different angles, as seen in the diagram at left. Any cross-section passing through the center of an ellipsoid forms an elliptic region, while the corresponding plane sections are ellipses on its surface. These degenerate to disks and circles, respectively, when the cutting planes are perpendicular to a symmetry axis. In more generality, the plane sections of a quadric are conic sections. A cross-section of a solid right circular cylinder extending between two bases is a disk if the cross-section is parallel to the cylinder's base, or an elliptic region (see diagram at right) if it is neither parallel nor perpendicular to the base. If the cutting plane is perpendicular to the base it consists of a rectangle (not shown) unless it is just tangent to the cylinder, in which case it is a single line segment. The term cylinder can also mean the lateral surface of a solid cylinder (see cylinder (geometry)). If a cylinder is used in this sense, the above paragraph would read as follows: A plane section of a right circular cylinder of finite length is a circle if the cutting plane is perpendicular to the cylinder's axis of symmetry, or an ellipse if it is neither parallel nor perpendicular to that axis. If the cutting plane is parallel to the axis the plane section consists of a pair of parallel line segments unless the cutting plane is tangent to the cylinder, in which case, the plane section is a single line segment. A plane section can be used to visualize the partial derivative of a function with respect to one of its arguments, as shown. Suppose . In taking the partial derivative of with respect to , one can take a plane section of the function at a fixed value of to plot the level curve of solely against ; then the partial derivative with respect to is the slope of the resulting two-dimensional graph. In related subjects A plane section of a probability density function of two random variables in which the cutting plane is at