For example, ("r", θ), ("r", θ+2π) and (−"r", θ+π) are all polar coordinates for the same point. The pole is represented by (0, θ) for any value of θ. There are two common methods for extending the polar coordinate system to three dimensions. In the cylindrical coordinate system, a "z"-coordinate with the same meaning as in Cartesian coordinates is added to the "r" and "θ" polar coordinates giving a triple ("r", "θ", "z"). Spherical coordinates take this a step further by converting the pair of cylindrical coordinates ("r", "z") to polar coordinates ("ρ", "φ") giving a triple ("ρ", "θ", "φ"). A point in the plane may be represented in "homogeneous coordinates" by a triple ("x", "y", "z") where "x"/"z" and "y"/"z" are the Cartesian coordinates of the point. This introduces an "extra" coordinate since only two are needed to specify a point on the plane, but this system is useful in that it represents any point on the projective plane without the use of infinity. In general, a homogeneous coordinate system is one where only the ratios of the coordinates are significant and not the actual values. Some other common coordinate systems are the following:  There are ways of describing curves without coordinates, using intrinsic equations that use invariant quantities such as curvature and arc length. These include:  Coordinates systems are often used to specify the position of a point, but they may also be used to specify the position of more complex figures such as lines, planes, circles or spheres. For example, Plücker coordinates are used to determine the position of a line in space. When there is a need, the type of figure being described is used to distinguish the type of coordinate system, for example the term "line coordinates" is used for any coordinate system that specifies the position of a line. It may occur that systems of coordinates for two different sets of geometric figures are equivalent in terms of their analysis. An example of this is the systems of homogeneous coordinates for points and lines in the projective plane. The two systems in a case like this are said to be "dualistic". Dualistic systems have the property that results from one system can be carried over to the other since these results are only different interpretations of the same analytical result; this is known as the "principle of duality".