# What Is the Y-Axis?

## What Is the Y-Axis?

The y-axis is the vertical axis on a graph.

The image below shows what we mean by the y-axis: Note: Only part of the y-axis is shown. It can extend forever up and down.

## How to Use the Y-Axis

Imagine you wanted to describe the position of a point on a graph. You would need to know how far across a point is and how far up a point is.

The y-axis lets us measure how far up a point is.

The y-axis is labelled with numbers (0, 1, 2, 3...) so you can measure how far up the point is. This gives the y-coordinate of a point, using Cartesian coordinates.

### Question

What is the y-coordinate of the point below? # 1

See how far up the y-axis the point is.

It can help to draw a line (in your minds eye or otherwise) from the point, straight across to the y-axis. We can see that the point is 4 units up the y-axis. The point has a y coordinate of 4.

## The Y-Axis Goes On Forever, Up and Down

In the images above, the y-axis starts at 0 and goes up to 5.

This can give the wrong impression. In fact, the y-axis can continue going up (past 100, past 1000, in fact forever).

The y-axis also goes below 0. It is labelled -1, -2, -3. It can also extend forever downwards. In practice, only draw the parts of the y-axis you need to show.

## The Y-Axis as a Number Line

The y-axis is a number line that extends vertically. ## The Equation of the Y-Axis

The equation of the y-axis is: It is an equation of a line.

This equation is true because the y-axis crosses the x-axis when x = 0. The x-coordinate of all points on the y-axis are 0