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Now that we've taken a detailed look at triangles, we can begin looking at a shape with an extra side and vertex: the quadrilateral. The word "quadrilateral" is composed of two main parts:

(1) quad - which means "four", and

(2) lateral - which means "side."

While triangles are very significant to the world around us, quadrilaterals are, perhaps, the most important and common type of polygon. For instance, quadrilaterals come in the form of numerous shapes, including squares, rectangles, rhombuses, kites, and trapezoids, just to name a few. Each of these shapes has their own defining characteristics, which we will learn about in the following lessons.

Take a look around your environment right now. A great number of the things you see are probably composed of some type of quadrilateral. Whether it's the computer screen you're looking at, the building you are in, or the sheets of paper you write on, quadrilaterals are everywhere. Let's explore the properties and characteristics of these special shapes to expand our knowledge of geometry.

## Polygons

Main Lesson: Polygons

Learn the different classifications of polygons, their names, and how we can use the Polygon Interior Angle Sum Theorem.

## Properties of Parallelograms

Main Lesson: Properties of Parallelograms

Become familiar with key terminology used to describe different parts of quadrilaterals, and study the properties that make parallelograms unique.

Main Lesson: Proving Quadrilaterals Are Parallelograms

Learn how to use the properties of parallelograms properly in two-column geometric proofs.

## Properties of Rectangles, Rhombuses, and Squares

Main Lesson: Properties of Rectangles, Rhombuses, and Squares

Discover the properties of three more types of parallelograms: rectangles, rhombuses, and squares.

## Properties of Trapezoids and Kites

Main Lesson: Properties of Trapezoids and Kites

Take a step away from parallelograms and learn about trapezoids and kites, whose opposite sides may intersect.