# Cartesian coordinate system

**Cartesian coordinate system**

**Video: Coordinate plane**

A **Cartesian coordinate system** is a coordinate system that specifies each point uniquely in a plane by a pair of numerical **coordinates**, which are the signed distances to the point from two fixed perpendicular directed lines, measured in the same unit of length. Each reference line is called a *coordinate axis* or just *axis* (plural *axes*) of the system, and the point where they meet is its *origin*, at ordered pair (0, 0). The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.

One can use the same principle to specify the position of any point in three-dimensional space by three Cartesian coordinates, its signed distances to three mutually perpendicular planes (or, equivalently, by its perpendicular projection onto three mutually perpendicular lines). In general, *n* Cartesian coordinates (an element of real *n*-space) specify the point in an *n*-dimensional Euclidean space for any dimension *n*. These coordinates are equal, up to sign, to distances from the point to *n* mutually perpendicular hyperplanes.

**Cartesian coordinate system with a circle of radius 2 centered at the origin marked in red.** The equation of a circle is (*x* − *a*)^{2} + (*y* − *b*)^{2} = *r*^{2} where *a* and *b* are the coordinates of the center (*a*, *b*) and *r* is the radius.

The invention of Cartesian coordinates in the 17th century by René Descartes (Latinized name: *Cartesius*) revolutionized mathematics by providing the first systematic link between Euclidean geometry and algebra. Using the Cartesian coordinate system, geometric shapes (such as curves) can be described by **Cartesian equations**: algebraic equations involving the coordinates of the points lying on 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.

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.

**Description**

**One dimension **

Choosing a Cartesian coordinate system for a one-dimensional space – that is, for a straight line—involves choosing a point *O* of the line (the origin), a unit of length, and an orientation for the line. An orientation chooses which of the two half-lines determined by *O* is the positive, and which is negative; we then say that the line "is oriented" (or "points") from the negative half towards the positive half. Then each point *P* of the line can be specified by its distance from *O*, taken with a + or − sign depending on which half-line contains *P*.

A line with a chosen Cartesian system is called a **number line**. Every real number has a unique location on the line. Conversely, every point on the line can be interpreted as a number in an ordered continuum such as the real numbers.

**Video: One Dimensional Coordinate System Worked Example 2**

**Two dimensions **

A Cartesian coordinate system in two dimensions (also called a **rectangular coordinate system** or an **orthogonal coordinate system**^{[6]}) is defined by an ordered pair of perpendicular lines (axes), a single unit of length for both axes, and an orientation for each axis. The point where the axes meet is taken as the origin for both, thus turning each axis into a number line. For any point *P*, a line is drawn through *P* perpendicular to each axis, and the position where it meets the axis is interpreted as a number. The two numbers, in that chosen order, are the *Cartesian coordinates* of *P*. The reverse construction allows one to determine the point *P* given its coordinates.

The first and second coordinates are called the *abscissa* and the *ordinate* of *P*, respectively; and the point where the axes meet is called the *origin* of the coordinate system. The coordinates are usually written as two numbers in parentheses, in that order, separated by a comma, as in (3, -10.5). Thus the origin has coordinates (0,0), and the points on the positive half-axes, one unit away from the origin, have coordinates (1,0) and (0,1).

In mathematics, physics, and engineering, the first axis is usually defined or depicted as horizontal and oriented to the right, and the second axis is vertical and oriented upwards. (However, in some computer graphics contexts, the ordinate axis may be oriented downwards.) The origin is often labeled *O*, and the two coordinates are often denoted by the letters *X* and *Y*, or *x* and *y*. The axes may then be referred to as the *X*-axis and *Y*-axis. The choices of letters come from the original convention, which is to use the latter part of the alphabet to indicate unknown values. The first part of the alphabet was used to designate known values.

**Video: Cartesian Coordinates in Two Dimensions**

**Three dimensions **

A three dimensional Cartesian .............

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