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Coordinates
Coordinates are an ordered set of numbers that define the position of a point. If the point is on a plane, then two numbers are used. To define the position of a point in three-dimensional space, we need three. On a plane (2D)To define the position of a point on a 2D plane, we use two numbers, called the x-coordinate and the y-coordinate. The x-coordinate tells where the point is in the left-right direction, and the y-coordinate tells where it is in the vertical, up-down direction. 
In the figure above the coordinates of the point A are 20, 15. The first (x) coordinate tells how far along the horizontal (x) axis it is, here 20. The second (y) coordinate tells how far up the vertical (y) axis it is, here 15. The x coordinate is always first in the pair. For more on this see Coordinates of a point on the plane. In space (3D)To define a point in 3D space we add a third coordinate called z. This tells how far the point is in the third dimension. You can think of this third (z) dimension as going in and out of the page.
Pages referring to 'coordinates'A method for finding the area of any polygon - regular, irregular, convex, concave if you know the coordinates of the vertices.
Description of how the position of a point can be defined by x and y coordinates.
Finding the distance between two points given their coordinates
Definiton of the equation of a straight line, in 'point - slope' form: y = m(x-Px) + Py
Definiton of the equation of a straight line, in 'slope and intercept' form: y = mx+b
How to find the intercept of a line, given the coordinates of two points on the line
Introduction to coordinate geometry.
Introduction to the concept of lines in coordinate geometry.
Definition of a line when the defining points are on the coordinate plane
Definition of a line segment when the defining points are on the coordinate plane
Finding the midpoint of a line segment given the coordinates of the endpoints
Definition of 'origin' and its relationship to coordinate geometry
Print blank graph paper - Cartesian (rectangular) coordinates
Definition of a ray when the defining points are on the coordinate plane
Definition of the slope of a line given the coordinates of two points on the line, includes slope as a ratio and an angle.
The coordinate plane defined with description of x,y axis, quadrants, origin.
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