How to Construct a Perpendicular Line to a Given Line Through Point on the Line

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Perpendicular lines are lines that form right angles. These types of lines are used often in geometry, so it is important to know how to construct them. The simplest way to construct a perpendicular line is to use a protractor. However, you will often be asked to construct a perpendicular line using only a compass and straightedge.

Steps

Using a Protractor

Line up the given line and the protractor. Set the protractor’s origin hole over the line's given point. Align the protractor so that its base line sits exactly over the given line. The origin hole is the small hole at the bottom-center of the protractor. The base line is the line on the bottom of the protractor marking 180/0 degrees.

Mark a point at the 90-degree line. This is the line measurement at the very top of the protractor. Two perpendicular lines form a 90-degree, or right, angle. Thus, to draw a perpendicular line, you need to draw a 90-degree angle.

Connect the given point with the 90-degree mark. Use the straightedge of the protractor to ensure that the line is straight. This line is perpendicular to the original line, through the given point on the line.

Using a Compass and Straight Edge

Draw an arc on either side of the given point. To do this, place the compass tip on the given point on the line. Then, swing the compass, drawing two arcs on both sides of the given point. The arcs should intersect the line. Mark and label the points where the arcs intersect the line. You can set the compass to any width for this step. For example, you might be given point A on a line. Use the compass to draw point P to the left, and point Q to the right.

Increase the width of the compass. Place the compass tip on the new point to the left of the original point. Stretch the compass so that it reaches about halfway between the original point, and the new point on the right. For example, place the compass tip on point P, and stretch the compass to a point about halfway between points A and Q.

Draw an arc above the line. You could also draw the arc below the line. Keeping the compass tip on the left point, swing the compass, drawing an arc. Make sure the arc crosses over the original point. For example, keep the compass tip on point P. Draw an arc above the line.

Draw a second arc, intersecting the first. Do not change the width of the compass. Set the compass tip on the right point. Swing the compass, drawing an arc above the line that intersects the first arc. Mark this intersection with another point. For example, place the compass tip on point Q. Draw an arc that intersects the first arc at point T.

Connect the given point to the point where the arcs intersect. Use a straightedge to ensure the line is straight. The line you draw is perpendicular to the first line, through the given point on the line. For example, draw a line connecting points A and T.