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In geometry, a **Cartesian coordinate system** (UK: /kɑːrˈtiːzjən/, US: /kɑːrˈtiʒən/) in a plane is a coordinate system that specifies each point uniquely by a pair of real numbers called *coordinates*, which are the signed distances to the point from two fixed perpendicular oriented lines, called *coordinate lines*, *coordinate axes* or just *axes* (plural of *axis*) of the system. The point where they meet is called the *origin* and has (0, 0) as coordinates.

Similarly, the position of any point in three-dimensional space can be specified by three *Cartesian coordinates*, which are the signed distances from the point to three mutually perpendicular planes. More generally, *n* Cartesian coordinates specify the point in an *n*-dimensional Euclidean space for any dimension *n*. These coordinates are the signed distances from the point to *n* mutually perpendicular fixed hyperplanes.

Cartesian coordinates are named for René Descartes, whose invention of them in the 17th century revolutionized mathematics by allowing the expression of problems of geometry in terms of algebra and calculus. Using the Cartesian coordinate system, geometric shapes (such as curves) can be described by equations involving the coordinates of points of the shape. For example, a circle of radius 2, centered at the origin of the plane, may be described as the set of all points whose coordinates *x* and *y* satisfy the equation *x*^{2} + *y*^{2} = 4; the area, the perimeter and the tangent line at any point can be computed from this equation by using integrals and derivatives, in a way that can be applied to any curve.

Cartesian coordinates are the foundation of analytic geometry, and provide enlightening geometric interpretations for many other branches of mathematics, such as linear algebra, complex analysis, differential geometry, multivariate calculus, group theory and more. A familiar example is the concept of the graph of a function. Cartesian coordinates are also essential tools for most applied disciplines that deal with geometry, including astronomy, physics, engineering and many more. They are the most common coordinate system used in computer graphics, computer-aided geometric design and other geometry-related data processing.