## Introduction: Constructing Perpendicular Lines

How to construct perpendicular lines using a compass and a straight edge.

### Teacher Notes

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## Step 1:

Draw a line with your straight edge and put two points on them. Label one A and the other one B.

## Step 2:

Draw an arch with your compass on B to the right.

## Step 3:

Draw an arch with your compass on B to the left.

## Step 4:

Draw an arch with your compass on A to the left. It should intersect with the arch you drew to the left on B.

## Step 5:

Draw an arch with your compass on A to the right. It should intersect with the arch you drew to the right on B.

## Step 6:

Connect the two intersection of the archs to create perpendicular lines.

## 2 Discussions

3 years ago

Not only will the new line be perpendicular to the first line

AB. It will bisect that line at it’s mid-point.

3 years ago

I learned this in 4th grade geometry back in the 70's, maybe in our computer graphics driven society this is no longer common knowledge. This process also determines the mid-point of your initial line. In a similar fashion, you can bisect an angle by placing your compass anywhere along one of the lines making up the angle and striking an arc and then repeating the process on the other line so the arcs intersect. Draw the bisection line from the vertex of angle to the intersection of the arcs.

On a large scale, this process can be used to layout athletic fields, like baseball diamonds and football fields allowing creation of accurate right angles. The lines are replaced by a string between two stakes. The compass is replaced by a long string with stakes on either end. You drive one stake into the ground and strike your arc with the other stake and the line pulled taut.