What is a parallelogram?

Let’s begin with defining some of the properties of a parallelogram.

1. A parallelogram is a four sided polygon.

2. The opposite sides of a parallelogram are parallel. So if you look at the picture below, sides AB and CD are parallel and AD and BC are parallel

3. The opposite angles of a parallelogram are congruent. So angles B and D are congruent and angles A and C are congruent. ( See picture above) For example if angle B is 120 degrees then angle D will also be 120 degrees. The same rule applies for angles D and B

4. The consecutive angles in a parallelogram are supplementary which means they add to 180 degrees. Therefore, if angle D measures 120 degrees, angle A will measure 60 degrees because they are consecutive angles and add up to 180 degrees.

5. In a parallelogram any two consecutive angles will be supplementary to each other. In the picture,

**A**and**B**,**B**and**C**,**C**and**D**, and**D**and**A**are all consecutive angles and therefore, are supplementary.
Let’s look at a practice problem. Given a parallelogram in which angle C is has a measure of 80 degrees. What are the other three angles?

1. Angle A will also be 80 degrees because it is opposite angle C.

2. If angle A is 80 degrees then angle B will be 100 degrees because they are consecutive angles.

3. Angle D will be 100 degrees because it is opposite angle B

For a visual explanation please watch the video

For more information on parallelograms see Parallelograms