Horizontal line test

In mathematics, one can use the horizontal line test in order to determine if a function is one-to-one or injective. The image below shows what a horizontal line test is supposed to look like.

Horizontal line test

The green line is the line that we use to do the test. It is basically a horizontal line that can cross or intersect a graph in at least 1 point.

In the image above, the line intersects the graph in two points and these points are (-1,-1) and (2, -1). 

Horizontal line test and one-to-one function

If a horizontal line test intersects the graph in more than one point, then the graph is not one-to-one. Therefore, the graph above is not one-to-one since the line intersects the graph in two points.

See below another horizontal line test that intersects the graph in only 1 point.

Horizontal line test

Here we draw two lines to emphasize the fact that no matter how many horizontal lines you draw, they will always cut or intersect this graph in only 1 point.

Since every horizontal line test will always cut the graph in only 1 point, we say that the graph immediately above is one-to-one.

What is the difference between a horizontal line test and a vertical line test?

The first graph we drew above is not one-to-one because the horizontal line crossed the two points (-1,-1) and (2, -1). As you can see here, two different x-values and these are - 1 and 2 are mapped to the same y-value or -1

When you do a horizontal line test, each x-value in the domain should be mapped to a different y-value in the range every time.

Other other hand, when you a vertical line test, each y-value in the range should be mapped to a different x-value in the domain.

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