Learn about Quadrilaterals and Their Parts In this lesson, let’s learn about quadrilaterals and their parts. How do we name the different parts of the quadrilateral given to us? So a quadrilateral means 4 sides. It’s a polygon with 4 sides. It has 4 sides, 4 angles. “Quad” means 4. Four sides, four angles, four vertices and I’ll point each one of these out and two diagonals. Well, here are the sides. As we know if this is the quadrilateral, this is a side, this is a side, this is a side, and this is a side. So what are the sides named? Well this side is DC or CD. So let’s go lower to higher CD. This side is BC, this side is AB and this side is AD. So sides are named by two letters which represent the end points or the vertices. What are the four angles? Well this is one angle, this is the second, this is the third, and this is the fourth. Naming the angles is easy. You just take a look at that vertex which is labeled A and that angle is called A so this is angle A, angle B, angle C, and angle D. This is the symbol for angle. The vertices are of course A, B, C, and D because that’s just the way they’re labeled. This is a vertex, this is a vertex and the plural of vertex is vertices. And these are two diagonals which connect the opposite vertices. So how do we name them? Well this is AC, and this is BD. So the two diagonals are AC and BD. So these are the parts of a quadrilateral. You’ve got sides, you’ve got angles, and you’ve got vertices and you’ve got diagonals. The sides are labeled by the labels of two endpoints, the two vertices. You put them together and that’s the name of the side AD. It’s this one.