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A rule for assigning to each point of a general space (that is to say: of a line segment, a surface, three-dimensional space or higher-dimensional analogues) or space-time a set of numbers for purposes of identification.

Many readers will know two examples from school: In the case of the line of real numbers, every point on the line corresponds to a real number which can be seen as its coordinate. What's important is that these coordinates reflect neighbourly relations: The number 1 lies between the number 0 and the number 2, and so does the point corresponding to it lie between the two points corresponding to 0 and 2. The second example is the usual X-Y-coordinate system (sometimes called Cartesian coordinates), by which every point in a plane can be characterized by two numbers: the first its X coordinate value, the second its Y coordinate value.

The examples reflect an important property of coordinates: To uniquely identify a point in space, one needs as many coordinate values as the space has dimensions.

Of the four coordinates defining an event in space-time, three serve to fix its location in three-dimensional space, while the fourth gives the point in time for the event.


  • coordinate system
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