Angles lie in a plane. To specify the point in space where an angle lies,
or where any figure exists, a plane can be assigned coordinates. Since a
plane is two-dimensional, only two coordinates are required to designate a
specific location for every point in the plane. One coordinate determines the
length, and the other determines width. In reality, length and width are the
same thing--they are used because they describe distance in two directions which
are perpendicular to each other. This is all
the coordinate plane is: a plane with two perpendicular axes by which
distance in either of two dimensions can be measured.
The coordinate plane consists of an origin and two axes. The origin is a
point. The axes are lines perpendicular to each other that intersect at the
origin. Below is pictured the coordinate plane, with the origin at point O.
Figure %: The coordinate plane
The origin is fixed, and designated as the point (0,0). Every other point is
assigned an ordered pair, (x, y), according to its position relative to the
origin. The two axes are named the $x$-axis and the y-axis. In most
drawings, the x-axis is the horizontal axis, and the y-axis is the vertical
axis, but this does not necessarily need to be the case. A point is assigned an
ordered pair consisting of two real numbers: The first is the x-coordinate,
which measures how far the point is from the y-axis. The second real number
making up an ordered pair is the y-coordinate, which measures the distance
between the point and the x-axis. Often the axes are pictured with tick marks
indicating length to make it easier to measure distance. When a point is drawn
into the coordinate plane and assigned an ordered pair, it is plotted. Take
a gander at the plotted points below.
Figure %: Some points plotted in the coordinate plane
Note that some of the coordinates are negative numbers. Negative distance does
not exist, but coordinates are given either positive or negative values to
specify which side of the given axis they are on. In most cases, the positive
direction of the x-axis points to the right, and the positive direction of the
y-axis points upward. Thus, for example, points on the left of the y-axis have
a negative x-coordinate. The positive directions don't always have to be these
directions, though. Often, as in the diagram above, the axes will only have an
arrow on the end which points in the positive direction. The other end has no
arrow. This is how one can tell where the positive and negative values lie.
A plane extends in all direction without limit. So does the coordinate plane.
Although there are many ways to draw the coordinate plane, it is always the same
thing: a point of origin and two axes, which intersect at the origin and lie
perpendicular to each other. The origin, by definition, always has the
coordinates (0,0). Every other point in the plane can be measured according to
the axes. Even the point (33563452143,23455434) exists and can be located in
any coordinate plane; it extends without limit. Below are some other ways to
draw the coordinate plane. All look different, but they are all the same
coordinate plane.
Figure %: Different views of the coordinate plane
The axes of the coordinate plane divide the plane into four regions--these
regions are called quadrants. The region in which the x-coordinate and the
y-coordinate are both positive is called Quadrant I. Quadrant II is the region
in which x < 0 and y > 0. Quadrant III is the region in which
x < 0 and y < 0. Quadrant IV is the region in which
x > 0 and y < 0. The quadrants are labeled in the figure below.
