The Cartesian plane is just a flat area where you can draw points, lines, shapes, or whatever takes your fancy. However, there are some things you need to know about before you just start drawing wily nily. First up, this area usually has two axes (axes is the plural form of axis) drawn on it, an ‘x-axis’ and a ‘y-axis’. These are just straight lines with markings along them. The ‘x-axis’ is a horizontal straight line, and the ‘y-axis’ is a vertical straight line which cuts through the x-axis. Here’s a diagram showing a Cartesian plane with these two axes:
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So if I want to tell someone to draw a point somewhere on the graph, I need to tell them where the point is relative to the numbers on the x and y-axes. For instance, say I wanted someone to draw a point where the cross is in the following graph. What you’d need to do is look where the cross lined up with on the x-axis and the y-axis. In this case, the cross lines up with ‘3’ on the x-axis, and ‘–4’ on the y-axis. These two numbers are called the x coordinate and y coordinate of the point.
Now you can usually write the coordinates of a point like this:
(x coordinate, y coordinate)
The x coordinate is always written first, followed by the y coordinate. So to write the position of our cross, we would write:
(3, –4)
On this graph notice how I’ve labelled the x and y-axes by putting an ‘x’ or ‘y’ at the end of the positive side of the axis. It’s a good idea to always label your axes so that the reader knows what each axis is measuring.
